VCA2612 BURR-BROWN | Alldatasheet
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Technical content
l LOW NOISE PREAMP:
- Low Input Noise: 1.25nV/√Hz
- Active Termination Noise Reduction
- Switchable Termination Value
- 80MHz Bandwidth
- 5dB to 25dB Gain
- Differential Input/Output l LOW NOISE VARIABLE GAIN AMPLIFIER:
- Low Noise VCA: 3.3nV/√Hz, Differential Programming Optimizes Noise Figure
- 24dB to 45dB Gain
- 40MHz Bandwidth
- Differential Input/Output l LOW CROSSTALK: 52dB at Max Gain, 5MHz l HIGH-SPEED VARIABLE GAIN ADJUST l SWITCHABLE EXTERNAL PROCESSING APPLICA TIONS l ULTRASOUND SYSTEMS l WIRELESS RECEIVERS l TEST EQUIPMENT VCA2612 International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111 Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132 © 2000 Burr-Brown Corporation PDS-1541B Printed in U.S.A. March, 2000 TM Dual, VARIABLE GAIN AMPLIFIER with Low Noise Preamp VCA2612 For most current data sheet and other product information, visit www.burr-brown.com Low Noise Preamp 5dB to 25dB Programmable Gain Amplifier 24 to 45dB Voltage Controlled Attenuator Analog Control Maximum Gain Select RF 2 RF 1 FB FBSW LNP INP LNP INN LNP GS1 LNP GS2 LNP GS3 LNP Gain Set Input LNP OUT P SELVCA INP LNP OUT N VCA INN VCA CNTLFB CNTL VCA OUT P VCA OUT N MGS 1 MGS 2 MGS 3 Maximum Gain Select VCA2612 (1 of 2 Channels)
DESCRIPTION
The VCA2612 is a highly integrated, dual receive chan- nel, signal processing subsystem. Each channel of the product consists of a low noise pre-amplifier (LNP) and a Variable Gain Amplifier (VGA). The LNP circuit provides the necessary connections to implement Active Termination (AT), a method of cable termination which results in up to 4.6dB noise figure improvement. Differ- ent cable termination characteristics can be accommo- dated by utilizing the VCA2612’s switchable LNA feed- back pins. The LNP has the ability to accept both differential and single ended inputs, and generates a differential output signal. The LNP provides strappable gains of 5dB, 17dB, 22dB and 25dB. The output of the LNP can be accessed externally for further signal processing, or fed directly into the VGA. The VCA2612’s VGA section consists of two parts, the Voltage Controlled Attenuator (VCA) and the Program- mable Gain Amplifier (PGA). The gain and gain range of the Programmable Gain Amplifier can be digitally programmed. The combination of these two program- mable elements results in a variable gain ranging from 0dB up to a maximum gain as defined by the user through external connections. The output of the VGA can be used in either a single-ended or differential mode to drive high performance analog-to-digital converters. The VCA2612 also features low crosstalk and outstand- ing distortion performance. The combination of low noise, and gain range programmability make the VCA2612 a versatile building block in a number of applications where noise performance is critical. The VCA2612 is available in a TQFP-48 package.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems. SPECIFICATIONS At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. VCA2612Y PARAMETER CONDITIONS MIN TYP MAX UNITS PREAMPLIFIER Input Resistance 600 k Ω Input Capacitance 15 pF Input Bias Current 1n A CMRR f = 1MHz, VCA CNTL = 0.2V 50 dB Maximum Input Voltage Preamp Gain = +5dB 1 Vp-p Preamp Gain = +25dB 112 mVp-p Input Voltage Noise(1) Preamp Gain = +5dB 3.5 nV/ √Hz Preamp Gain = +25dB 1.25 nV/ √Hz Input Current Noise Independent of Gain 350 fA/ √Hz Noise Figure, RS = 75Ω , RIN = 75Ω (1) R F = 550Ω , PreAmp Gain = 22dB, 6.2 dB PGA Gain = 39dB Bandwidth Gain = 22dB 80 MHz PROGRAMMABLE VARIABLE GAIN AMPLIFIER Peak Input Voltage Differential 2 Vp-p –3dB Bandwidth 40 MHz Slew Rate 300 V/ µs Output Signal Range R L ≥ 500Ω Each Side to Ground 2.5 ±1V Output Impedance f = 5MHz 1 Ω Output Short-Circuit Current ±40 mA Third Harmonic Distortion f = 5MHz, V OUT = 1Vp-p, VCACNTL = 3.0V –45 –71 dBc Second Harmonic Distortion f = 5MHz, V OUT = 1Vp-p, VCACNTL = 3.0V –45 –63 dBc IMD, Two-Tone V OUT = 2Vp-p, f = 1MHz –80 dBc VOUT = 2Vp-p, f = 10MHz –80 dBc 1dB Compression Point f = 5MHz, Output Referred, Differential 6 Vp-p Crosstalk VOUT = 1Vp-p, f = 1MHz, Max Gain Both Channels 68 dB Group Delay Variation 1MHz < f < 10MHz, Full Gain Range ±2n s ACCURACY Gain Slope 10.9 dB/V Gain Error ±1(2) dB Output Offset Voltage ±50 mV GAIN CONTROL INTERFACE Input Voltage (VCACNTL ) Range 0 to 3.0 V Input Resistance 1M Ω Response Time 45dB Gain Change, MGS = 111 0.2 µs POWER SUPPLY Specified Operating Range 4.75 5.0 5.25 V Power Dissipation Operating, Both Channels 410 475 mW NOTE: (1) For preamp driving VGA. (2) Referenced to best fit dB-linear curve.
This integrated circuit can be damaged by ESD. Burr-Brown recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. ESD damage can range from subtle performance degrada- tion to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published specifications. ABSOLUTE MAXIMUM RATINGS PACKAGE SPECIFIED DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT PRODUCT PACKAGE NUMBER RANGE MARKING NUMBER (1) MEDIA VCA2612Y TQFP-48 Surface Mount 355 –40 °C to +85°C A12 VCA2612Y/250 Tape and Reel NOTE: (1) Models with a slash (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K indicates 2000 devices per reel). Ordering 2000 pieces of “VCA2612Y/2K” will get a single 2000-piece Tape and Reel. PACKAGE/ORDERING INFORMATION
2 NC Do Not Connect
3 NC Do Not Connect
4 VCA INNA Channel A VCA Negative Input
5 VCA INPA Channel A VCA Positive Input
6 LNP OUT NA Channel A LNP Negative Output
7 LNP OUT PA Channel A LNP Positive Output
8 SWFBA Channel A Switched Feedback Output
9 FBA Channel A Feedback Output
10 COMP1A Channel A Frequency Compensation 1
11 COMP2A Channel A Frequency Compensation 2
12 LNP INNA Channel A LNP Inverting Input
13 LNP GS3 A Channel A LNP Gain Strap 3
14 LNP GS2 A Channel A LNP Gain Strap 2
15 LNP GS1 A Channel A LNP Gain Strap 1
16 LNP INPA Channel A LNP Noninverting Input
17 V DD R +Supply for Internal Reference
18 V BIAS 0.01µF Bypass to Ground 19 V CM 0.01µF Bypass to Ground
20 GNDR Ground for Internal Reference
21 LNP INPB Channel B LNP Noninverting Input
22 LNP GS1 B Channel B LNP Gain Strap 1
23 LNP GS2 B Channel B LNP Gain Strap 2
24 LNP GS3 B Channel B LNP Gain Strap 3
25 LNP INNB Channel B LNP Inverting Input
26 COMP2B Channel B Frequency Compensation 2
27 COMP1B Channel B Frequency Compensation 1
28 FBB Channel B Feedback Output
29 SWFBB Channel B Switched Feedback Output
30 LNP OUT PB Channel B LNP Positive Output
31 LNP OUT NB Channel B LNP Negative Output
32 VCA INPB Channel B VCA Positive Input
33 VCA INNB Channel B VCA Negative Input
34 NC Do Not Connect
35 NC Do Not Connect
36 V DD B Channel B +Analog Supply
37 GNDB Channel B Analog Ground
38 VCA OUT NB Channel B VCA Negative Output
39 VCA OUT PB Channel B VCA Positive Output
40 MGS 3 Maximum Gain Select 3 (LSB)
41 MGS 2 Maximum Gain Select 2
42 MGS 1 Maximum Gain Select 1 (MSB)
43 VCA CNTL VCA Control Voltage
44 VCA INSEL VCA Input Select, HI = External
45 FBSW CNTL Feedback Switch Control: HI = ON,
46 VCA OUT PA Channel A VCA Positive Output
47 VCA OUT NA Channel A VCA Negative Output
48 GNDA Channel A Analog Ground
PIN DESIGNATOR DESCRIPTION PIN DESIGNATOR DESCRIPTION PIN DESCRIPTIONS VDD B NC NC VCA INNB VCA INPB LNP OUT NB LNP OUT PB SWFBB FBB COMP1B COMP2B LNP INNB GNDA VCA OUT NA VCA OUT PA FBSW CNTL VCA INSEL VCA CNTL MGS 1 MGS 2 MGS 3 VCA OUT PB VCA OUT NB GNDB LNP GS3 A LNP GS2 A LNP GS1 A LNP INPA VDD R VBIAS VCM GNDR LNP INPB LNP GS1 B LNP GS2 B LNP GS3 B VDD A NC NC VCA INNA VCA INPA LNP OUT NA LNP OUT PA SWFBA FBA COMP1A COMP2A LNP INNA 48 47 46 45 44 43 42 41 40 39 38 13 14 15 16 17 18 19 20 21 22 23 VCA2612
TYPICAL PERFORMANCE CURVES At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. GAIN vs VCACNTL VCA CNTL (V) Gain (dB) MGS = 111 MGS = 110 MGS = 101 MGS = 100 MGS = 011 MGS = 010 MGS = 001 MGS = 000 GAIN ERROR vs TEMPERATURE VCA CNTL (V) Gain Error (dB) 2.0 1.5 1.0 0.5 –0.5 –1.0 –1.5 –2.0 +25°C –40°C +85°C GAIN ERROR vs VCA CNTL VCA CNTL (V) Gain Error (dB) 2.0 1.5 1.0 0.5 –0.5 –1.0 –1.5 –2.0 10MHz 1MHz 5MHz GAIN ERROR vs VCA CNTL VCA CNTL (V) Gain Error (dB) 2.0 1.5 1.0 0.5 –0.5 –1.0 –1.5 –2.0 MGS = 011MGS = 000 MGS = 111 GAIN MATCH: CHA to CHB = 0.2V Delta Gain (dB) Units 100 GAIN MATCH: CHA to CHB = 3.0V Delta Gain (dB) Units 100
TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. GAIN vs FREQUENCY (Pre-Amp) Frequency (MHz) 0.1 1 10 100 Gain (dB) LNP = 25dB LNP = 22dB LNP = 17dB LNP = 5dB GAIN vs FREQUENCY (VCA and PGA, VCACNTL = 0.2V) Frequency (MHz) 0.1 1 10 100 Gain (dB) 5.0 4.0 3.0 2.0 1.0 0.0 –1.0 –2.0 –3.0 –4.0 –5.0 MGS = 111 MGS = 100 MGS = 011 MGS = 000 GAIN vs FREQUENCY (VCA and PGA, VCACNTL = 3.0V) Frequency (MHz) 0.1 1 10 100 Gain (dB) MGS = 111 MGS = 100 MGS = 011 MGS = 000 GAIN vs FREQUENCY (VCA CNTL = 3.0V) Frequency (MHz) 0.1 1 10 100 Gain (dB) LNP = 25dB LNP = 22dB LNP = 5dB LNP = 17dB GAIN vs FREQUENCY (LNP = 22dB) Frequency (MHz) 0.1 1 10 100 Gain (dB) VCA CNTL = 3.0V VCA CNTL = 1.6V VCA CNTL = 0.2V OUTPUT REFERRED NOISE vs VCA CNTL VCA CNTL (V) Noise (nv/√Hz) 1800 1600 1400 1200 1000 800 600 400 200 R S= 50Ω MGS = 111 MGS = 011
TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. INPUT REFERRED NOISE vs R S R S (Ω ) 1 10 100 1000 Noise (nV√Hz 10.0 1.0 0.1 NOISE FIGURE vs RS (VCA CNTL = 3.0V) R S (Ω ) 10 100 1000 Noise Figure (dB) NOISE FIGURE vs VCA CNTL VCA CNTL (V) Noise Figure (dB) INPUT REFERRED NOISE vs VCA CNTL VCA CNTL (V) Noise (nV/√Hz) MGS = 011 MGS = 111 R S= 50Ω LNP vs FREQUENCY (Differential, 2Vp-p) Frequency (MHz) 0.1 1 10 100 Harmonic Distortion (dBc) –45 –50 –55 –60 –65 –70 –75 –80 3rd Harmonic 2nd Harmonic LNP vs FREQUENCY (Single-Ended, 1Vp-p) Frequency (MHz) 0.1 1 10 100 Harmonic Distortion (dBc) –45 –50 –55 –60 –65 –70 –75 –80 2nd Harmonic 3rd Harmonic
TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. HARMONIC DISTORTION vs FREQUENCY (Differential, 2Vp-p, MGS = 000) Frequency (MHz) 0.1 1 10 Harmonic Distortion (dBc) –40 –45 –50 –55 –60 –65 –70 –75 –80 –85 –90 VCA CNTL = 0.2V, H2 VCA CNTL = 0.2V, H3 VCA CNTL = 3.0V, H2 VCA CNTL = 3.0V, H3 HARMONIC DISTORTION vs FREQUENCY (Differential, 2Vp-p, MGS = 011) Frequency (Hz) 0.1 1 10 Harmonic Distortion (dBc) –40 –45 –50 –55 –60 –65 –70 –75 –80 –85 –90 VCA CNTL = 0.2V, H2 VCA CNTL = 0.2V, H3 VCA CNTL = 3.0V, H2 VCA CNTL = 3.0V, H3 HARMONIC DISTORTION vs FREQUENCY (Differential, 2Vp-p, MGS = 111) Frequency (MHz) 0.1 1 10 Harmonic Distortion (dBc) –30 –35 –40 –45 –50 –55 –60 –65 –70 –75 –80 VCA CNTL = 0.2V, H2 VCA CNTL = 0.2V, H3 VCA CNTL = 3.0V, H2 VCA CNTL = 3.0V, H3 HARMONIC DISTORTION vs FREQUENCY (Single-Ended, 1Vp-p, MGS = 000) Frequency (MHz) 0.1 1 10 Harmonic Distortion (dBc) –40 –45 –50 –55 –60 –65 –70 –75 –80 –85 –90 VCA CNTL = 0.2V, H2 VCA CNTL = 0.2V, H3 VCA CNTL = 3.0V, H2 VCA CNTL = 3.0V, H3 HARMONIC DISTORTION vs FREQUENCY (Single-Ended, 1Vp-p, MGS = 011) Frequency (MHz) 0.1 1 10 Harmonic Distortion (dBc) –40 –45 –50 –55 –60 –65 –70 –75 –80 –85 –90 VCA CNTL = 0.2V, H2 VCA CNTL = 0.2V, H3 VCA CNTL = 3.0V, H2 VCA CNTL = 3.0V, H3 HARMONIC DISTORTION vs FREQUENCY (Single-Ended, 1Vp-p, MGS = 111) Frequency (MHz) 0.1 1 10 Harmonic Distortion (dBc) –30 –35 –40 –45 –50 –55 –60 –65 –70 –75 –80 –85 VCA CNTL = 0.2V, H2 VCA CNTL = 0.2V, H3 VCA CNTL = 3.0V, H2 VCA CNTL = 3.0V, H3
INTERMODULATION DISTORTION (Differential, 2Vp-p, f = 10MHz) Frequency (MHz) 9.989.96 10 10.2 10.4 Power (dBm) –15 –25 –35 –45 –55 –65 –75 –85 –95 –105 TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. –1dB COMPRESSION vs VCA CNTL VCA CNTL (V) PIN (dBm) –10 –15 –20 –25 –30 –35 –40 3rd-ORDER INTERCEPT vs VCA CNTL VCA CNTL (V) IP3 (dBm) –10 –15 –20 –25 –30 –35 –40 –45 –50 INTERMODULATION DISTORTION (Single-Ended, 1Vp-p, f = 10MHz) Frequency (MHz) 9.989.96 10 10.2 10.4 Power (dBm) –15 –25 –35 –45 –55 –65 –75 –85 –95 –105 HARMONIC DISTORTION vs VCA CNTL (Differential, 2Vp-p) VCA CNTL (V) Harmonic Distortion (dBc) –45 –50 –55 –60 –65 –70 –75 –80 MGS = 000, H2 MGS = 011, H2 MGS = 111, H2 MGS = 000, H3 MGS = 011, H3 MGS = 111, H3 HARMONIC DISTORTION vs VCA CNTL (Single-Ended, 1Vp-p) VCA CNTL (V) Harmonic Distortion (dBc) –45 –50 –55 –60 –65 –70 –75 –80 MGS = 000, H2 MGS = 011, H2 MGS = 111, H2 MGS = 000, H3 MGS = 011, H3 MGS = 111, H3
TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. CMRR vs FREQUENCY (LNP only) Frequency (MHz) 0.1 1 10 100 CMRR (dB) –10 –20 –30 –40 –50 –60 –70 –80 PULSE RESPONSE (BURSTS) (Differential, VCACNTL = 3.0V, MGS = 111) 200ns/div Output 500mV/div Input 10V/div OVERLOAD RECOVERY (Differential, VCACNTL = 3.0V, MGS = 111) Input 1V/div Output 1V/div 200ns/div GAIN RESPONSE (Differential, VCACNTL Pulsed, MGS = 111) Output 500mV/div Input 2V/div 100ns/div CMRR vs FREQUENCY (VCA only) Frequency (MHz) 0.1 1 10 100 CMRR (dB) –10 –20 –30 –40 –50 –60 –70 –80 –90 VCA CNTL = 0.2V VCA CNTL = 1.4V VCA CNTL = 3.0V CROSS TALK vs FREQUENCY (Single-Ended, 1Vp-p, MGS = 011) Frequency (MHz) 0.1 1 10 Cross Talk (dB) –10 –20 –30 –40 –50 –60 –70 –80 –90 VCA CNTRL = 0V VCA CNTRL = 1.5V VCA CNTRL = 3.0V
TYPICAL PERFORMANCE CURVES (Cont.) At TA = +25°C, VDD = 5V, load resistance = 500Ω on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted. The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended unless otherwise noted. 79.5 78.5 77.5 76.5 ICC vs TEMPERATURE Temperature (°C) –40 –10 5–25 35 50 20 65 80 95 ICC (mA) GROUP DELAY vs FREQUENCY Frequency (MHz) 1 10 100 Group Delay (ns) VCA CNTL = 3.0V VCA CNTL = 0.2V –45 –40 –35 –30 –25 –20 –15 –10 PSRR vs FREQUENCY Frequency (Hz) PSRR (dB)
each circuit block are provided in the following sections. distortion performance with very high input impedance. Q10 play the same role for signals on the –IN side. FIGURE 4. Schematic of the Low Noise Pre-Amplifier (LNP). composite gain control characteristic of the entire VCA2612. 59µs at the lowest gain (25dB).
The LNP is capable of generating a 2Vp-p differential signal. The maximum signal at the LNP input is therefore 2Vp-p divided by the LNP gain. An input signal greater than this would exceed the linear range of the LNP, an especially important consideration at low LNP gain settings. ACTIVE FEEDBACK WITH THE LNP One of the key features of the LNP architecture is the ability to employ active-feedback termination to achieve superior noise performance. Active feedback termination achieves a lower noise figure than conventional shunt termination, essentially because no signal current is wasted in the termi- nation resistor itself. Another way to understand this is as follows: Consider first that the input source, at the far end of the signal cable has a cable-matching source resistance of R S. Using conventional shunt termination at the LNP input, a second terminating resistor of value RS is connected to ground. Therefore, the signal loss is 6dB due to the voltage divider action of the series and shunt R S resistors. The effective source resistance has been reduced by the same factor of 2, but the noise contribution has been reduced by only the √2, only a 3dB reduction. Therefore, the net theoretical SNR degradation is 3dB, assuming a noise-free amplifier input. (In practice, the amplifier noise contribution will degrade both the unterminated and the terminated noise figures, somewhat reducing the distinction between them.) Figure 5 shows an amplifier using active feedback. This diagram appears very similar to a traditional inverting am- plifier. However, the analysis is somewhat different because the gain “A” in this case is not a very large open-loop op amp gain; rather it is the relatively low and controlled gain of the LNP itself. Thus, the impedance at the inverting amplifier terminal will be reduced by a finite amount, as given in the familiar relationship of Equation (3): where R F is the feedback resistor (supplied externally be- tween the LNPINP and FB terminals for each channel), A is the user-selected gain of the LNP, and RIN is the resulting amplifier input impedance with active feedback. In this case, unlike the conventional termination above, both the signal voltage and the R S noise are attenuated by the same factor of It is also possible to create other gain settings by connecting an external resistor between LNPGS1 on one side, and LNPG S2 and/or LNPG S3 on the other. In that case, the internal resistor values shown in Figure 4 should be com- bined with the external resistor to calculate the effective value of R S for use in Equation (1). The resulting expression for external resistor value is given in Equation (2). where REXT is the externally selected resistor value needed to achieve the desired gain setting, RS1 is the fixed parallel resistor in Figure 4, and RFIX is the effective fixed value of the remaining internal resistors: RS2, RS3 or (RS2 || RS3) depending on the pin connections. Note that the best process and temperature stability will be achieved by using the pre-programmed fixed gain options of Table I, since the gain is then set entirely by internal resistor ratios, which are typically accurate to ±0.5%, and track quite well over process and temperature. When combining exter- nal resistors with the internal values to create an effective R S value, note that the internal resistors have a typical tempera- ture coefficient of +700ppm/°C and an absolute value toler- ance of approximately ±5%, yielding somewhat less predict- able and stable gain settings. With or without external resistors, the board layout should use short Gain Strap connections to minimize parasitic resistance and inductance effects. The overall noise performance of the VCA2612 will vary as a function of gain. Table II shows the typical input-and output-referred noise densities of the entire VCA2612 for maximum VCA and PGA gain; i.e., VCA CNTL set to 3.0V and all MGS bits set to “1”. Note that the input-referred noise values include the contribution of a 50Ω fixed source impedance, and are therefore somewhat larger than the intrinsic input noise. As the LNP gain is reduced, the noise contribution from the VCA/PGA portion becomes more significant, resulting in higher input-referred noise. How- ever, the output-referred noise, which is indicative of the overall SNR at that gain setting, is reduced. NOISE (nv/√Hz) LNP GAIN (dB) Input-Referred Output-Referred 25 1.54 2260 22 1.59 1650 17 1.82 1060 5 4.07 597 TABLE II. Noise Performance for MGS = 111 and VCACNTL = 3.0V . LNP PIN STRAPPING LNP GAIN (dB) LNPG S1, LNPGS2, LNPGS3 Connected Together 25 LNPG S1 Connected to LNPGS3 22 LNPG S1 Connected to LNPGS2 17 All Pins Open 5 TABLE I. Pin Strappings of the LNP for V arious Gains. (3) (2) R R AIN F= +()1 where RL is the load resistor in the drains of Q3 and Q8, and R S is the resistor connected between the sources of the input transistors Q4 and Q7. The connections for various RS combinations are brought out to device pins LNPGS1, LNPGS2 and LNPG S3 (pins 13-15 for channel A, 22-24 for channel B). These Gain Strap pins allow the user to establish one of four fixed LNP gain options as shown in Table I. To preserve the low noise performance of the LNP, the user should take care to minimize resistance in the input lead. A parasitic resistance of only 10Ω will contribute 0.4nV/√Hz. R R R R R Gain R R Gain R REXT S L FIX L S FIX SL = +•
signal handling at both input and output. compatibility with the VCA input. source voltage and VT is the threshold voltage of the FET. S and the parallel FET network.
- In the simplified diagram of Figure 10, each shunt FET is
the stepped attenuation options. FIGURE 10. Programmable Attenuator Section.
FIGURE 9. Piecewise Approximation to Logarithmic Control Characteristics.