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ISO124, ISO122 isolation amplifier circuit

ISO124 and ISO122 are unity gain isolation amplifier IC offer by Burr-Brown and TEXAS Instruments. Older isolation amplifiers modules were very costly and big in size and weight. While ISO124 available in SOIC28 and DIP16 package and low cost. Both IC’s consist on two circuits isolated by an 2pF differential capacitive barrier with PWM transmission across barrier. It require only few external components without complexity. Maximum Input signal voltage are +/-10V for ISO124 and +/-12V for ISO122. and maximum supply voltages are +/-4.5 to +/- 18V.

ISO124 exhibit flat unity gain for DC to 10KHz with 0.01% linearity error and maximum 0.5% gain error. However, gain dropped down to -3dB at 50KHz input signal.

Leakage 100uA at 1500V 10KHz is sufficient insulation for AC operation. Here is 110V AC measuring circuit with ISO124 circuit with pin configurations. R1,R2 and R3 are down converting AC 110V to 2V which are acceptable input range of ISO124.

Pin 1,2,15 and 16 are part of input circuit isolated from output circuit. Pin 1 and pin 2 are used for +/-12V supply of input and pin 16 is common point of +/-12V power supply and reference for input signal Vin pin#15. Input signal can be positive or negative with respect to GND1

Pin 10 and 9 are +/-12V output side power supply with respect to GND2 @ Pin8. While Pin7 is output signal Vout with respect to GND2. Output may positive either negative according to input signal. And there is no phase difference between input and output signal.

Note:- Following circuit shows DIP16 package pin number SOIC28 pin numbers are (1,2,13,14, 15,16,27 and 28) however no difference in orientation.

Important Design Considrations

  • Both input and output power supply voltages should be same.
  • ISO124 and ISO122 require dual(+V and -V) supplies for both isolated sides.
  • Input impedance is about 200K-Ohm. You may use FET-input Op-Amp to avoid input signal loading.
  • Input power supply should be isolated DC source either transformer based or an DC/DC Converter. However if Isolation is not required then following pins can be shorted +V1 with +V2, -V1 with -V2 and GND1 with GND2. Circuit will work as normal voltage follower.
  • In PCB no copper track should be pass under center-area of IC.
  • 1uF tantalum capacitor is recommend for each supply pin with respect to GND1 and GND2.
  • If input or output circuit contains high voltage then with stand voltage should be considered in case of AC signal. Due to isolation barrier consist on 2pF capacitors which may pass some AC current.
ISO124P isolation amplifier by Texas Instruments

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