THAT Corporation Design Note 138

1 THAT Corporation s 1510 and 1512 are low noise, wide bandwidth microphone preamplifiersavailable in several different industry-standard packages. They allow designers to upgradeexisting designs to take advantage of the superior performance of these new ICs. Both partsinclude on-board laser-trimmed resistors which allow differential gain to be set with a singleexternal resistor. They differ from each other primarily in their internal gain structure, andtheir resulting noise 1510 is a pin-for-pin, drop-in replacement for the Analog Devices ssm2019 , and thenow discontinued SSM2017, as well as the Texas Instruments INA217 audio 1510 noise and gain characteristics are equal to or better than those of these otherparts, but the 1510 also improves on their distortion, bandwidth, and slew 1512 is also a pin-for-pin replacement for the ssm2019 /2017 and INA217, but due toits different gain structure, produces significantly lower noise at low gains and requiresdifferent external resistors to reach the same gain.

2 Neither the 1510 nor the 1512 duplicatesthe front-end output terminals available on the TI INA163, but in instances when theseoutputs are not used, which is often true in audio applications, the 1512 can improve signifi-cantly on the INA163 noise performance (as well as on distortion, bandwidth, and slew rate)while producing a similar gain characteristic without other circuit modifications. It is possi-ble to gain the noise advantages of the 1512 in substitution for the ssm2019 /2017 and theINA217, but the resulting gain characteristics may be different enough to require othercircuit modifications for some Design note is intended to offer some guidance on gain control to designers who areconsidering the 1510 or 1512, first for new designs, but also for replacements in GainAs shown in Figure 1, the 1510 and 1512use a three-amplifier instrumentation ampli-fier (IA) topology.

3 This configuration has theadvantage of not amplifying common modevoltages as it amplifies signal, and as such,one can achieve excellent common moderejection ratios (CMRR) that increase withthe gain setting. Additionally, the input stage of theseparts uses an unconventional input amplifiers are configured to providecurrent feedback via resistors RA and RB tothe emitters of the input transistors. Thesenodes are brought out to pins RG1 and allows the differential gain of the inputsection to be controlled by the external resis-tor RG. In order to reduce input voltage noise at low gains, the configuration of the 1512 differsfrom that of the 1510. The 1512 output stage has 6 dB less gain than that of the 1510. Thisleads to different gain equations for the two parts as the 1510: , or .AV=1+10k RGRG=10k AV 1 Copyright 2005 - 2011 Design Note 138by THAT CorporationPage 1 of 8 All rights reserved45 Sumner St; Milford, MA 01757 USA; ; 600064 Rev 00 GConfiguring Gain with the THAT1510 & THAT1512 THAT Corporation Design Note 138V+V--InRG2RG1+InOutRef+-5k5k(10k)5k5k (10k)5k5kOutput StageInput StageRARBRG-AV-AVFigure 1.

4 1510/1512 Equivalent Circuit(THAT1512 values shown in parentheses).And, for the 1512: , or ; where AV is the voltage gain, and RG isAV= +5k RGRG=5k AV gain equations for the 1510 are identical to those for the SSM2017, ssm2019 , andthe INA217. This makes the 1510 convenient for drop-in replacement of these parts, offeringa true second source with improved performance, and without requiring any redesign gain equations for the 1512 are unique. Because of this, existing circuits may requiresome adjustment to substitute the 1512 for a 1510 or other manufacturers preamp ICs. Table 1 compares the gain of the 1510 with that of the 1512 for various values of RG, whileTable 2 shows RG for various desired gains for both Potentiometer-Based Variable Gain ControlFigure 2 shows a simplified application circuit for the 1510 and 1512 using a potentiome-ter to control gain.

5 This circuit, and all the others shown in this Design note, focuses ongain control to the exclusion of other necessary features. For example, it does not show theinput RFI protection, phantom power and associated fault protection, input ac-coupling, orpower supply bypassing essential to implement a mic preamp with ICs available on theTHAT Corporation Design Note 138 Configuring Gain with the THAT 1510 & 1512 Copyright 2005 - 2011 Design Note 138by THAT CorporationPage 2 of 8 All rights reserved45 Sumner St; Milford, MA 01757 USA; ; 600064 Rev 00 GFigure 2. Simplified application circuit for the 1510/1512+ dB+ dB5 + dB+ dB10 + dB+ dB50 + dB+ dB100 + dB+ dB500 + dB+ dB1 k + dB+ dB5 k 0 dB+ dB10 k dB 1512 Gain1510 GainRGTable 1. Gain (rounded to the nearest ) vs RGforthe 1510 and 10 +60 dB16 32 +50 dB50 101 +40 dB161 327 +30 dB526 1,111 +20 dB1,878 4,625 +10 dB3,344 10,048 +6 dB10,000 0 dB Not possible-6 dB1512 RG1510 RGGainTable 2.

6 RG(rounded to the nearest 1 ) vs Gain forthe 1510 and today. For complete information on circuit configurations for these ICs, see THAT s1510/1512 data 3 shows how the decibel gain of the circuit of Figure 2 varies with pot rotationusing a 1510 and various 10 k pots for RGV. The pots are all reverse audio (log) taper, usinga two-segment piece-wise linear approximation to the log function; the tapers vary to 20%. The curves assume negligible wiper- and end-resistance for RGV. RGF is 10 . RGFlimits the minimum resistance between RG1 and RG2 to 10 , which limits the maximum gainto ~ 60 dB. RGV, at 10 k, sets the maximum resistance between RG1 and RG2 to ~ 10 k, whichlimits minimum gain to ~ 6 dB for the pot may look the most nonlinear on the plot, but in actuality, it is the best fitfor a straight line approximation between 6 dB and 60 dB.

7 With the 20% taper pot, the last30 degrees of rotation results in over a 30 dB change in gain from ~28 dB to 60dB,whereas the taper has only a 15 dB variation in gain (from ~45 dB to 60 dB) over thissame angle. This makes settings much less sensitive at high gains. Figure 4 shows the same family of curves, but for the 1512. In this case, RGV remains 10k , but RGF is 5 ; for the 1512, this limits maximum gain to approximately 60 dB. Notethat with the 10 k pot, the 1512 reaches a minimum of 0 dB gain. Also, the wider gain rangeincreases the sensitivity of gain change vs. rotation in the last 30 degrees of rotation (athigher gains). Note that the 1512 will produce identical curves of gain vs. pot setting as those ofFigure 3 if RGV were 5 k , and RGF were 5 .SSM2017 / ssm2019 / INA217 to 1510 The 1510 is a direct replacement for the SSM2017, ssm2019 , and INA217.

8 No changesare required in existing circuits to take advantage of the improved distortion, bandwidth,and (in some cases) noise performance of the 1510 compared to these other Corporation Design Note 138 Configuring Gain with the THAT 1510 & 1512 Copyright 2005 - 2011 Design Note 138by THAT CorporationPage 3 of 8 All rights reserved45 Sumner St; Milford, MA 01757 USA; ; 600064 Rev 00G0306090120150180210240270300 Rotation (in deg)0102030405060 Gain(in dB) (in deg)0102030405060 Gain(in dB) 3. Gain trajectories for the circuit of Fig. 2, 1510 with various pot tapers, RGV=10k , RGF=10 .Figure 4. Gain trajectories for the circuit of Fig. 2, 1512 with various pot tapers, RGV=10k , RGF=10 .INA163 to 1512 - Without Any Circuit ChangesWhile the 1512 and the INA163 have different gain equations, for many applications the1512 may be directly substituted for the INA163.

9 In such cases, noise at low gains can besignificantly improved, and the characteristic of gain vs. pot rotation is only slightlychanged. With this substitution, noise athigh gain will be nearly identical, but noise atminimum gain will improve for the 1512compared to the gain equations for the INA163 are: , or , which areAV=1+6k RGRG=6k AV 1similar to those for the 5 shows an example applicationcircuit for the INA163. RGV is a 2 k , 5%reverse audio taper pot ( , the Alps taper). RGF is 5 . With theseresistances, gain varies from +12 to +62 , at 6800 uF, avoids changes in dc outputoffset with gain. Note that VO1 and VO2 (theinput stage outputs at pins 1 and 14,respectively), are not shown in Figure 6, the 1512 may besubstituted directly for the INA163 in thiscircuit. Figure 7 shows the gain trajectoriesof these two circuits along with the differ-ence in gain that results from this directsubstitution.

10 At low gains there is dB of error, and at higher gains, theerror drops to about dB. Figure 8 shows the theoretical frontpanel scaling for these two circuits. Theprimary difference between the two gaintrajectories is a relatively constant ~2 dB. Ifthis error is unacceptable, it could beadjusted by changing the gain of a subse-quent Corporation Design Note 138 Configuring Gain with the THAT 1510 & 1512 Copyright 2005 - 2011 Design Note 138by THAT CorporationPage 4 of 8 All rights reserved45 Sumner St; Milford, MA 01757 USA; ; 600064 Rev 00 GVccVeeRefV+V-In-In+OutR12k2R22k2C16800u /10 VRGFRGVRG1 Vin-Vin+VoCWRG2U1 INA163Vo1Vo2 VccSen5R2kVccVeeRefV+V-In-In+OutR12k2R22 k2C16800u/10 VRGFRGVRG1 Vin-Vin+VoCWRG2U1 THAT1512 Vcc2k5 RFigure 5. Example application circuit for the INA163,+12 ~+60 dB 6. Substituting a 1512 for an INA163 inthe circuit of Figure (in deg) Gain(dotted)THAT1512 GaindB GaindB DifferenceFigure 7.