2920

2920 ETC1

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Technical content

a DESCRIPTION ANDO INSTRUCTIONS . . COS: Optical Electronics Un

20 MHz BIPOLAR LOGARITHMIC AMPLIFIER

a FEATURES APPLICATIONS ¢ WIDE BANDWIDTH: 20MHz * LOG FUNCTION ¢ WIDE DYNAMIC RANGE: 80 dB « ULTRASONIC MEASUREMENTS ¢ LOG FUNCTION * SONAR ¢ CURRENT AND VOLTAGE INPUTS ° VIDEOLOG ee

DESCRIPTION

The 2920 bipolar logarithmic amplifier is de- signed to perform logarithmic functions on the signals applied to its inputs. Its specifications make it most suitable to a variety of applications, The hybrid design, packaged ina 24 pin Dual-In- NY ig Line Package, allows use of standard sockets e ov pe. and takes up only a minimum of PC board real nas | estate. 4. The 2920 has been designed to fillthe gap that 2% 1 ‘3 exists for logarithmic amplifiers, where wide iy a tile available bandwidth and wide dynamic range at Rea \\ 2 Gale high frequencies are concerned.Most amplifiers ge) » ae of this kind have wide bandwidth and good iss. +143 dynamic range at frequencies near DC. The ide Say - A dynamic range of the device covers a minimum oe of 4 decades {80dB). Over 75% of this range (60dB) a logarithmic error of only +0.7% is typical, with a maximum error of only +3% at the . oe upper portion of the dynamic range. The fre- be scaled to any requirements with the addition quency response registers a full 1 to 20 MHz and of an operational amplifier. the 2920 can therefore be used for a variety of 5 video applications. y The 2920 can easily be compensated with the application of a single capacitor. Because of the Another advantage of the design of the 2920 is influence of this capacitor on the dynamic range the fact, that, although the device is current and frequency response, performance can easily driven, appropriate scaling resistors, applied be shaped to fit even filtering applications. As externally or by use of the built-in resistor, allow would be expected an increase in capacitance it lo become a voltage driven amplifier. decreases the bandwidth of the device, The power supply requirements are standard The 2920 finds applications in video and audio £15 volt inputs, and range from +5 volts to +20 compression circults and any other place where volts. The low power consumption, even at the a logarithmic function is required. The wide maximum rated output, make the device energy frequency range also makes it ideal for ultra- efficient, The £100 to +400 mV output range can sonic measurements and in sonar devices.

Specifications at Tp = +25°C. Veg = +1SVDC unless otherwise noted. MODEL 2920 | PARAMETER Tin tye max units DYNAMIC RANGE Ee ee RATED OUTPUT Voltage +100 +400 mv Current 41 mA Dynamic Resistance 3 fl Output Coefficient 75 mV Temperature Coefficient -16 mvc Dynamic Range +3x107 A Dynamic Range 23.0 mv Resistance Current Input 1 a Resistance Voltage Input 1000 a Polarity Bipolar INPUT OFFSET VOLTAGE Initial Offset ADJ Drift 2100 uve INPUT BIAS CURRENT Initial Bias +30 nA Drift nar MAXIMUM LOGARITHMIC ERROR 80dB Dynamic Range 1 60d8 Dynamic Range 0.7 FREQUENCY RESPONSE 10KHz 80 a8 100KHz 80 dB MHz 50 a8 3MHz 40 dB 10MHz 30 6B 20MHz 20 dB TEMPERATURE RANGE Thermal Resistance of Package cw Quiescent Temperature Rise °C Operating +85 °C Storage +150 °C POWER SUPPLY Rated Voltage +56 +16 +20 v Current Quiescent +12/-10 mA ——— The information in this publication has been carefully checked specifications are subject to change wilhout notice. No patent and is believed to be reliable; however, na responsibility is rights are granted to any of the circulls described herein. assumed for possible inaccuracies or omissions. Prices and

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17 OUTPUT

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There are some misconceptions regarding the between current and voltage exhibited by a pn operation and function of logarithmic amplifi- junction, ers. To explain this a bit better, the following tigure is provided. The relationship of current through the diode . and the voltage across it can be expressed by, INPUT WAVEFORM FOR LOGARITHMIC CONVERSION Vik 4 ig=lg(e' -1) where ig current through diode a lo reverse current Vevoltage across diode 30m and where the factor K is dependent on junction material and temperature. If the diode is con- anv nected in the feedback-loop of an operational amplifier, the input current becomes iy and the output voltage becomes V+ as shown in the dia- 300W gram, below if it is between 3 x 10% and 3 x 107 Amp. Since the input current if is equal to Viq/R the The vertical axis shows an input signal voltage equation for the diode becomes: of a given frequency. The horizontal axis indi- Vi cates a time axis. No particular scale is shown Vo = -Kin 2 + Kin lo because this axis is only representative. The vol- R tage is a positive going signal that uses a +300. offset with alternative spikes ranging to 2920 EQUIVALENT 3mV and 3V. It must be remembered that the LOG ELEMENT logarithm of 0 is not defined and if a signal is L_] suspected to go to a zero value, a small positive DC offset must be introduced. it Vino oVouts- lf one assumes that a 30dB dynamic range is “ used, the 3V input signal will be represented by a 400mV output. . . For large enough output voltages, the exponen- The 3mV signal is then 3 decades down from the tial expression becomes very much larger than 1 3V which translates to a 225mV difference so that and the equation can be simplified. If the natural the output appears as 400mV—225mV = 175mV. logarithm is taken on both sides and the equa- tion is rearranged, it reads: The advantage of the OEI devices is their ability 7 to do logarithmic conversions at frequencies to Vo = -Kin Vin + Kin Ig 20MHz. Devices with outstanding DC character- R istics abound on the market, but none is capable of operations at higher frequencies. This shows that the output voltage is propor- tional to the logarithm of the input voltage, plus a constant. The constant K as well as the inverse INTRODUCTION current Ig are constant for a constant tem- The 2920 bipolar logarithmic amplifier can be perature. employed when a logarithmic function is re- quired. Since the device is current driven, the input voltage to the device must be converted to APPLICATIONS . a current by use of internal or external resistors. The transfer function and accuracy limitations of Input current levels range from 300 nanoamps to the 2920 are given below. 3 milliamps and this 80dB range provides output 2 Vin voltages between 100 and 400 millivolts with an $x10° ASe “<9mA output coefficient of 75 mV/decade. The output Vin in follows the ideal logarithmic function with a high Vo = —Kin — + Kin Ig accuracy up to 20 MHz for small signals. Rin +100mV = Vo <t400mVv THEORY OF OPERATION The 2920 is basically an operational amplifier K = 0326 at 25°C with diodes in its feedback loop. Thus, advan- tage is taken of the exponential relationship lo = 1.4 10% Amp

As is shown, the logarithmic relationship holds BASIC CONNECTIONS, for Vo between 300 nanoamps to 3 milliamps. TYPICAL APPLICATIONS. The constant Kinlg is approximately .6V at 25° C The diagram in figure 1 shows the 2920 in the and may be subtracted to the offset adjustment. basic log connection. Pins 5 and 7 provide the input terminals and can be used for current or voltage sources. Voltage sources between +0.3 millivolts to+3 volts can make use of the internal Due to the temperature sensitivity of the pn resistor of 1K ohm junction K is temperature sensitive. In the trans- fer function, K is proportional to the temperature o-v in degrees Kelvin. As a result, for a given input req current, the output voltage will vary as a function of temperature. This behavior must also be taken vounse, 5 a SOT into account, and appropriate means of com- La pensation must be designed into the circuit. In 2920 Las the section with typical performance curves, 2 3 ie. data can be found for this purpose. For a+25°C temperature swing, the output voltage can vary + + as much as t35mV., Hur | Se ow > o- When multiplying logarithmically the constant K FIGURE 1: BASIC CONNECTIONS will be additive and thus must be taken into account when designing with the 2920. Division available at pin 7. Pin 17 is the output connec- of two numbers cancels the constant automati- tion. As mentioned before, the offset adjustment cally. Conversion to logarithms of other bases potentiometer, connected between pins 9, 10 can be accomplished by appropriate gain ad- and the negative power supply is mandatory. If justment of the following stages. “this adjustment is not available the dynamic range of the circuit will suffer. Ceo is used to com- Loa 1c OUTPUT pensate for load capacitance. o-V OFFSET ADY. ks ce, “Vv ° ° p 9 0 ° INPUT! 5.7 = 2920 24 2 For a logarithmic conversion use pin 17 to pro- vide the output voltage Vp. A 1K ohm potenti- yr | wer ometer (trimpot) is connected between pins 9 v= -V and 10. This offset adjustment pot must be included in any design with the 2920. The input FIGURE 2: BASIC LOGARITHMIC CONNECTIONS can be applied to either pins 5 or7.1f an external Seen Gee or the inputis a current, connec- The diagram of figure 2 is, as the previous one, jon can be made to pin 5. However, for conven- A A ience, the 2920 also contains a 1K ohm resistor. designed a8 #.logarithmic amplifier. As‘shown ’ . there, input signals are applied to pins 5 or7 with the same voltage ranges. This diagram differs from the one in figure 1 In that a compensation . . capacitor Cy, is connected between pins 5 and The offset voltage caused by some mismatchin —_ 4g. polarity must be observed. This capacitor will the logarithmic elements, is small and can be overcompensate the 2920 and thus reduce the adjusted by applying a bipolar signal to the bandwidth the device is capable of providing. In Input. The offset can then be adjusted, until the most applications, this measure is not necessary output swings to equal magnitude in both the and actually detrimental to the overall perfor- positive and negative direction (dynamic trim). mance. However, when the circuit is particularly noisy, Cc, will better the noise performance of the 2920.