# instrumentation amplifier with [2 opamp + ADC] vs [3 opamp + ADC]

I have an 16bit ADC able to do differential measurement with range -5V to +5V (ADS1115), do you think both circuits would work?

It's required that the adc input is within -5V and +5V range, else it would fry.

I think the 3-OPAMP circuit has the advantage that output would be always -5..+5, so no risk for the ADC. I would use a basic OP-07C opamp for this.

Note: I didn't draw the RC circuits.

• Why build a discrete instrumentation amplifier out of op amps when you can buy an integrated instrumentation amplifier? – Null Aug 5 '19 at 14:28
• If your opamps have +/-5 V rails, then the output won't be above 5 V. The datasheet for the OP07C shows the peak output voltage with 15 V rails as 13 V, which suggests you'd get +/- ~3 V with 5 V rails. – Colin Aug 5 '19 at 14:30
• @Null you are right, but where I live there are no in-amp with low bias current (I need few fA), but I have opamp with low bias current. – Vinlar Aug 5 '19 at 14:35
• @Colin good point, I need higher voltage for +Vs and -Vs at the OP07, I would use +7.5V and -7.5V rails, so output would fine in range. – Vinlar Aug 5 '19 at 14:37

It's required that the adc input is within -5V and +5V range, else it would fry.

That's not quite true.

You should be able to capitalize on the following part of the data sheet specification: -

Input current, continuous (any pin except power supply pins) –10 to +10 mA


Given also that the input range is: -

Analog input voltage  GND – 0.3 V to VDD + 0.3 V


You have a good scope to implement a current limit (and therefore voltage limit) by using a series resistor on your input pins. For instance, if the peak voltage at your input could be as high as (say) 8 volts, a current limit resistor of 1 kohm would produce a current into the ADC input of (8-5.3) volts / 1000 ohms = 2.7 mA i.e. significantly below the 10 mA limit.

I'm not saying you do not need an InAmp to get good circuit performance, but you don't necessarily need one for protecting the ADC.

• Dear Andy, I do have a 10K resistor (and 68uF as RC filter) at the input of the ADC - not represented. So input current cannot exceed 0.5mA, far below the limit. You suggest it could work ok only with 2 opamp+ADC? As written in the comment, I can't find inamp with only few fA of input bias current but opamp are easy to find. – Vinlar Aug 5 '19 at 15:17
• I'm saying that you don't need an InAmp to get protection you think you might need. – Andy aka Aug 5 '19 at 15:39
• ok, as long there is the 10K in serie with the input of the ADC, it's protected. So both schematics are equal, better for me to not bother with the additional OP07. – Vinlar Aug 5 '19 at 15:42

You add current limiting resistor on opamp output and diode clamps.

simulate this circuit – Schematic created using CircuitLab

• Last time I used clamping diode (1N4148), I noticed the output is modified once Opamp output is near but below clamping voltage. I suppose there is a safe range to use where output isn't affected by the clamping diodes. If clamping is -5/+5V, what is working range where output isn't affected? (-4 to +4?) – Vinlar Aug 5 '19 at 17:42
• @Vinlar You clamp the voltage to the ADC input range, the supply of opamp is higher you said +/-7.5V – Marko Buršič Aug 6 '19 at 8:43