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0 HIE ES SCs alll DATA AND SPECIFICATIONS
DESCRIPTION AND INSTRUCTIONS EB Optical Electronics Incorporated eee
10 MHz BIPOLAR LOGARITHMIC AMPLIFIER
e WIDE BANDWIDTH: 10 MHz e AUDIO COMPRESSION ¢ WIDE DYNAMIC RANGE: 80 dB ¢ LOG ¢ ANTILOG *LOG ¢ ANTILOG * ULTRA SONIC MEASUREMENTS ¢ CURRENT AND VOLTAGE INPUTS ¢ SONAR * VIDEO LOG RIPTION ‘ DESC! fe] a ase The 2910 bipolar logarithmic amplifier is designed (sag to perform logarithmic or antilogarithmic func- ye tions on the signals applied to its inputs. Square aah root functions can also be implemented. Its Q@ ‘ agi specifications make it most suitable to a variety FA oO of applications. The hybrid design, packaged ina b a Ce; 16 pin Dual-In-Line Package, allows use of stan- » A . 291g dard sockets and takes up only a minimum of PC a “ board real estate. tb y » The 2910 has been designed to fill the gap that 4 exists for logarithmic and antilogarithmic ampli- fiers, where wide available bandwidth and wide The low power consumption, even at the maximum dynamic range at high frequencies is concerned, rated output, make the device energy efficient, Most amplifiers of this kind have wide bandwidth The £100m¥ to+400 mVolts output range can be and good dynamic range at frequencies near DC. scaled to any requirements with the addition of a The dynamic range of the device covers a minl- simple operational amplifier. mum of 4 decades (80 db). Over 75% of this range . 7 {60 db) alogarithmic error of only+0.7% is typical, The 2910 can easily be compensated with the with a maximum error of only £3% at the upper application of a single capacitor. Because of the portion of the dynamic range. The frequency influence of this capacitor on the dynamic range response registers a full 1 to 10 MHz and the 2910 and frequency response, performance can easily can therefore be used for a variety of video be shaped to fit even filtering applications. As applications. would be expected, an increase in capacitance decreases the bandwidth of the device. Another advantage of the design of the 2910 is the .: — ve fact, that, although the device is current driven, The 2910 finds applications in video and audio appropriate scaling resistors, applied externally compression circuits and any other place where a or by use of the built-in resistors, allow it to logarithmic orantilogarithmic function is required. become a voltage driven amplifier. Even square root signal processing can easily be accomplished. The wide frequency range also The power supply requirements are standard £15 makes it ideal for ultrasonic measurements and in volt inputs, and range from +5 volts to £20 volts. sonar devices,
Specifications at Ta = +28°€, Veg = +1SVDC unless otherwise noted. MODEL 2910 [PARAMETERS MIN =] tye Tmax [units RATED OUTPUT Voltage +100 2400 mv Current +1 mA Dynamic Resistance 3 a2 Output Coefficient 75 mV//decade Temperature Coefficient “1.6 mveC Dynamic Range +3x107 +3x107 A Dynamic Range 20,0001 +10 Vv Resistance Current Input 1 n Resistance Voltage Input 1K 10K 2 Polarity Bipolar INPUT OFFSET VOLTAGE Initial Offset ADJ Orift avec INPUT BIAS CURRENT Initial Bias +20 pa Orift +500 nArc MAXIMUM LOGARITHMIC ERROR 80dB Dynamic Range 1 60d8 Dynamic Range 0.7 FREQUENCY RESPONSE 1KHz 80 dB 10KHz 80 6B 100KHz 80 dB 1IMHz 60 dB 3SMHz 40 a8 1OMHz 20 6B TEMPERATURE RANGE Thermal Resistance of Package 60 c/w Quiescent Temp Rise 22 °c Operating +85 °c Storage +125 °c POWER SUPPLY Rated Voltage 356 215 +20 v Current Quiescent 2412 mA MECHANICAL DESCRIPTION: The 2910 is Tormey hth a standard 16-pin DIP. Its pins are compatible P9090 9 on al with standard 0.3-inch dual in-line sockets. The . ab oy case is made of glass-fiber-filled diallylphthalate z ° and is filled with epoxy encapsulant. + ances vyyyvoud —— 1 . sb tr TTR a, oe atesean | 3-v Supply 12 30k Voge inpua | en | 4tog Eamern 13.Ourrentinput__| ok | oa
9 Input Oftsat 169¥ Supe ee 4
TYPICAL PERFORMANCE CURVES (Ta = +25°C, Veg = 15VDC unless otherwise noted) FREQUENC' 0s TEMPERATURES INPUT OFFSET VOLTAGE DRIFT VS. INPUT CURRENT F rar a= a ~~ z a 3 Ca wt | Tg, A 5 LAY | bd ~~ % Pa s 7 4 Teste ie [~~ awa z / | vessy ~ _ erry Wve ‘300vA Sma JG tewrenatune Pe 5 m0 ) ttuweratune co) * ed INPUT CURRENT NC OURRENT POS TEMPERATURE vINeUT VOLTAGE m vs TEMSTRATURE eH LE } Ft tt | CN oe eat DST] i Y) | Saco N ult AY YY | A | = . NS \\7 7 = — i > Z g ae > E IZ ; dof | | | TSN] =} : “Terry 7 Lt de TT a E 73 so 8 Teweenarune SI 7% 180 sed ee ba Laced ad TEMPERATURE cc! * There are some misconceptions regarding the If one assumes that a 30d8 dynamic range is operation and function of logarithmic ampli- used, the 3V input signal will be presented by a fiers. To explain this a bit better, the following 400m¥ output. figure is provided. The 3mV si Fs gnal is then 3 decades down from pte the 3V which translates to a 225mV difference so that the output appears as 400mV — 225mvV = i 175mv. wow The advantage of the OE! devices is their abil- po ity to do logarithmic conversions at frequen- ] cies to 10MHz. Devices with outstanding DC char- acteristics abound on the market, but none is sv T] ann | | capable of operations at higher frequencies. Por The vertical axis shows an input signal voltage : “ a of a given frequency. The horizontal axis indi- o a cates a time axis, No particular scale is shown 7? © ye TYPICAL because this axis is only representative. The ai BOARD voltage is a positive going signal that uses a ee TOL i LAYOUT +300, offset with alternative spikes ranging bh Z to 3mV and 3V. It must be remembered that the 3 logarithm of 0 is not defined and if a signal is & suspected to go to a zero value, a small : positive OC offset must be introduced.
The 2910 bipolar logarithmic amplifier can be : employed whenever logarithmic or antilogarith- t mic functions are required. Since the device is Vino oVout=-Ve current driven, the input voltage to the device must be converted to a current by use of internal or external resistors. Input current levels range from 300 nanoamps to 3 milliamps and this 80dB Instead of inserting the logging element in the range provides output voltages between 100 and feedback-loop it can be connected to the input of 400 millivolts with an output coefficient of 75 the operational amplifier. The feedback resistor mV/decade. The output follows the ideal log- Ry is then used. With this arrangement, shown arithmic function with a high accuracy up to 10 below, the appropriate equations become: MHz for small signals. i= Ine VK 24 THEORY OF OPERATION L if = Ioe ° el Vo (4) The 2910 is basically an operational amplifier '_=— with a log element in its feedback loop, or at the ! Rf (5) device input. Thus advantage is taken of the ig exponential relationship between current and _ 4 voltage exhibited by a pn junction. Depending Vino we Vout on the connection scheme used, logarithmic or f antilogarithmic functions can be obtained with v, the same device. for3 x 10-7 Amp <—2<3x10° Amp The relationship of current through the log ele- Rr ment and the voltage across it can be expressed by, . Since the current i¢ equals -lg and the input \\¢ voltage Vjpj, equals V+ the equation becomes: if= lo (e VK -1) —o— _ 41 Vin/K (6) VE 1) Vg = -Ril (e Vin/K 4) Vin/K -, where i¢ current through loging element For the Leal of e ie < 1 the equation Ip reverse current becomes: V4 voltage across loging element Vo =-Rflge Vin/K {?) and where the factor K is dependent on junction o material and temperature. If the log element is Which shows that the output voltage is the connected in the feedback-loop of an opera- antilogarithm of the input voltage. tional amplifier, the input current becomes ig and the output voltage becomes V;, as shown in the ae below, if it is between 3x10-? and 3x10- APPLICATIONS Since the input current if is equal to Vin/R the The transfer function and accuracy limitations of equation for the loging element becomes the 2910 are given below: =8 Vi Vin 2 ga 1, te Vo/K (2) : 3x10 mac—l< ama R fo lol : Vo = -K In “i+ K In Io Rin For large enough output voltages the exponen- Rin 100MV = Vo = 400mv tial expression becomes very much larger than 1 K = 0326 at 25°C and the equation can be simplified. If the natural logarithm is taken on both sides and the equa- Ig = 1.4 X 10-8 Amp tion is rearranged it reads: Vin As is shown, the logarithmic relationship holds Vo =7Kin — + Kin lo {3) for Vg between 300 nanoAmps to 3 milliamps. A The constant K In I is approximately .6 Volts at 25°C and may be subtracted by the offset This shows that the output voltage is propor- adjustment. tional to the logarithm of the input voltage, plus a constant. The constant K as well as the inverse Oue to the temperature sensitivity of the pn current Ig are constant for a constant junction K is temperature sensitive. In the trans- temperature. fer function K is proportional to the temperature
in degrees Kelvin. As a result, for a given input These resistors can also be used as feedback current, the output voltage will vary as a function resistors in the antilog mode, Here pin 1 provides of temperature. This behavior must also be taken the output voltage. Feedback is accomplished into account, and appropriate means of compen- via an external resistor to pin 13 or by use of sation must be designed into the circuit. In the either of the internal resistors on pin 12 or 14 section with typical performance curves, data respectively. The offset adjustment pot of 1K can be found for this purpose. For a +25°C ohm is also mandatory for this use, and it is temperature swing, the output voltage can vary connected between pins 9 and 10. as much as +35mV, Se r A The offset voltage, caused by some mismatch in When multiplying logarithmically the constant K the logarithmic elements, is small and can be will be additive and thus must be taken into adjusted by applying a bipolar signal to the input. account when designing with the 2910, Division The offset can then be adjusted, until the output of two numbers cancels the constant automati- swings to equal magnitude in both the positive cally. Conversion to logarithms of other bases and negative direction. can be accomplished by appropriate gain adjust- ments of the following stages. Depending on the connections chosen the 2910 will either produce BASIC CONNECTIONS, an output that is proportional to the logarithm of TYPICAL APPLICATIONS. the input, or an output that is proportional to the . antilogarithm of the input as shown below: The diagram in figure 1 shows the 2910 in the basic log connection. Pins 12, 13, or 14 provide 45 the input terminals and can be used for currentor OFFSET ADJ maa o-18v 3 10 oe Lover [ is] aa INPUT 230g 9% Oy oourTeut ° I] fs ie 4 ° 2910 2910 L = 16 3.8 aan O+i6V 6-1sv OFFSET ADJ ma FIGURE 1: BASIC CONNECTIONS 9 10 voltage sources. The input voltage range for pin Vino 4 | Wout 12 is+3 millivolts to 30 volts for a+ 15 volt power 2910 supply. Voltage sources between + 0.3 millivolts to +3 volts can make use of the 10 times smaller 13 internal resistor of 1K ohm available at pin 14, Pins 1 and 4 must be tied together to make the output connection, As mentioned before, the offset adjustment potentiometer, connected be- Antteguitienis ouput tween pins9, 10 and the negative power supply is . . mandatory. If this adjustment is not available the For a logarithmic output pins 1 and 4 are tied dynamic range of the circuit will suffer, together, and provide the output voltage Vo. A 1K ohm potentiometer (trimpot) is connected be- The diagram of figure 2 is, as the previous one, tween pins 9 and 10. This offset adjustment pot must be included in any design with the 2910. ce O-18V The input can be applied to either pins 12, 13, or 14. Ifan external resistor is used, or the inputisa mite current, connection can be made to pin 13. s B39 10 However, for convenience, the 2910 also contains mete aeons ' ae CUTRUT a 1K ohm anda 10K ohm resistor. Either one can 2910 4 be used for a voltage input. L eee = 6 38 ‘The information in this publication has been caretully checked and is B believed to be reliable: Nowever. NO responsibility is assumed for possible inaccuracies or omvissions, Prices and specitications ve d+isv b-isv the circus deteribed herein ene meareors ™ FIGURE 2: BASIC LOGARITHMIC CONNECTIONS
designed as a logarithmic amplifier, As shown Figure 5 is essentially a repetition of the diagram there, input signals are applied to pins 12, 13 or of figure 4 except that the input is series con- 14 with the same voltage ranges. This diagram nected to the signal source via a resistor. This differs from the one in figure 1 in that a compen- then allows the source to be a voltage source. RA sation capacitor C, is connected between pins must be selected to keep input currents inside 13 and 15. Polarity must be observed. This the specified limits. capacitor will overcompensate the 2910 and thus ov reduce the bandwidth the device is capable of ma providing. In most applications, this measure is VOLTAGE Rin * 10 not necessary and actually detrimental to the Meure TI oouTPUT overall performance, However, when the circuit 2910 4 is particularly noisy, Cg will better the noise ir performance of the 2910. 16. is The basic antilogarithm connections are given in ‘ev the diagram of figure 3, As shown, the input and a aa! output pins are essentially reversed. The input FIGURE 5: VOLTAGE INPUT CONNECTIONS Throughout these discussions, mention has been o-s¥ made of the importance of the offset compensa- ey tion of circuits employing the 2910. Figure 6 : = shows a circuit designed for cases where very INPUT © + 4 OQUTPUT accurate offset adjustments must be made. The
2910 Horlt LJ °- Sv
© INPUT o ia Jy ooureuT z i 2910 busy b-iny E FIGURE3: BASIC ANTI-LOGARITHMIC CONNECTION 6 7 3a ae signal is applied at pin 4 and input voltages can . LY range from+ 100 millivolts to+ 400 millivolts, Pin ow twee 1 together with pin 12 or pin 14 provide the output. When high accuracy is required, pin 12 2 ¥ should be used, and for widest bandwidth, pin 14 5. < will serve best. Here as in the other circuits, use crssen of the 1K ohm trimpot is mandatory, particularly Monetoctured by O.tyF at lower signal levels. Figure 4 shows a 2910 el a application when a current source is available as FIGURE 6; STABILIZED BIPOLAR OR LOW DUTY input. Currents can 0-19 CYCLE PULSE LOGARITHMIC AMPLIFIER [ *‘] circuit is a rivet circuit that allows coarse adjust- 3 rr) ment via the offset pot, which is connected INPUT O s 7 OOUTPUT between pins 9, 10 and the negative supply ce! 2910 4 voltage. The input is again connected at pin 14. ‘6 The output taken off pins 1 and 4, isted backviaa * 7 3 8 1 Megohm resistor to the input of the LF 356H + [| operational amplifier. The .1 «F capacitor shorts ~ any possible AC voltage to ground, thus letting dal ead this input follow the average DC offset of the , input. The other operational amplifier input is FIT ICT TD Ce Dato held at virtual ground via the 1 Megohm resistor. range from + 3 Amps to 3 milliamps for a full The output of this amplifier is connected to pin 12 range of output voltages. If the current source is of the 2910 and thus provides soft offset correc- used, the bypass capacitor C_ may be necessary tions automatically. This circuit is recommended with values from 10pF to .001 uF. As with all 2910 when the dynamic range capability of the 2910 applications, a compensation potentiometer be- should be fully taken advantage of, for example, tween the pins 9, 10, and negative supply is when processing bipolar or low duty cycle pulse required. signals. Optical Electronics Incorporated P.O. Box 11140 «Tucson. Arizona 85734 ¢ TWX-910-952-1283 * Ph, 602-624-8358 OFS4A-B310M