I had made an circuit to convert a 35mV differential signal with a Common mode voltage of 4.5V to a 35mV signal with respect to ground [Gain=1]. I used AD620 for this purpose. AD620 is operated between -12V and +12V dc supply. I am able to get 35mV output from AD620. The problem is that there is a momentary spike of 2Volts [~ 1sec] generated when i switch off the 24v adapter supply. This is observed even when the input is not given to AD620. I tried following things but not able to rectify the issue (1) Change 24V Dc adapter (2) Connect a capacitor between positive and negative power rails (3) Connect a capacitor between Vout and GND of AD620 (4) re-wiring the whole circuit in a different breadboard (5) Change capacitor values for TC7662. The circuit diagram is attached.

 

there is a momentary spike of 2Volts [~ 1sec] generated when i switch off the 24v adapter supply.

Why is this a problem?
Why are you concerned about the signal when the adapter is switched off?

 

Why is this a problem?
Why are you concerned about the signal when the adapter is switched off?

There is an ADC ADS1156 at the output of AD620 which got burnt due to this spike

 

Whats your expected max input to the ADS1256?

Spec says -0.3 - AVDD+0.3v is safe on analog i/p, but you need to limit input current to ADS1156 to < 10mA. Whats your desired conversion rate?

[edit]Not finding datasheet for ads1156, do you mean ads1256 or ads1158?

 

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Whats your expected max input to the ADS1256?

Spec says -0.3 - AVDD+0.3v is safe on analog i/p, but you need to limit input current to ADS1156 to < 10mA. Whats your desired conversion rate?

[edit]Not finding datasheet for ads1156, do you mean ads1256 or ads1158?

Sorry Its ads1115 module. It was operated at 5V DC with gain=16

 

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Sorry Its ads1115 module. It was operated at 5V DC with gain=16

OK, no matter as specs are same for the range, common TI technology.

Put a 4k7 resistor in series with the AD620 output - this will limit input surge to << 10mA when AVDD goes to 0v. The spike is probably due to the shutdown time of your TC7662.

Is there a reason you're using +/- 12v for AD620 given you're using a gain of 1?

 

OK, no matter as specs are same for the range, common TI technology.

Put a 4k7 resistor in series with the AD620 output - this will limit input surge to << 10mA when AVDD goes to 0v. The spike is probably due to the shutdown time of your TC7662.

Is there a reason you're using +/- 12v for AD620 given you're using a gain of 1?

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I used a voltage regulator which is already available with me in my box [and >CMV of input]. No other specific reason to chose those values.

This is a trial circuit I made in breadboard before going for actual circuit.The details of actual circuit I am planning to build is given below:

The circuit I finally wants to make has to accepts 0-200mV at a common mode voltage of 10V. So first I planned to delete the CMV from the Whetstone bridge signal using AD620 and then feed it to ADs1115 . As per diamond plot tool of Analog Instruments I have to supply +15/-15V to AD620 for a common mode voltage of 10V.

 

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I used a voltage regulator which is already available with me in my box [and >CMV of input]. No other specific reason to chose those values.

This is a trial circuit I made in breadboard before going for actual circuit.The details of actual circuit I am planning to build is given below:

The circuit I finally wants to make has to accepts 0-200mV at a common mode voltage of 10V. So first I planned to delete the CMV from the Whetstone bridge signal using AD620 and then feed it to ADs1115 . As per diamond plot tool of Analog Instruments I have to supply +15/-15V to AD620 for a common mode voltage of 10V.

Fair enough.

 

I’d recommend you use something like this to generate the +12V, rather than a 7812 and 7662.

https://www.ebay.co.uk/itm/124408231408?var=425278334388

 

ok

 

OK, no matter as specs are same for the range, common TI technology.

Put a 4k7 resistor in series with the AD620 output - this will limit input surge to << 10mA when AVDD goes to 0v. The spike is probably due to the shutdown time of your TC7662.

Is there a reason you're using +/- 12v for AD620 given you're using a gain of 1?

Tried 4K7 resistor. But voltage spike is still there

 

Tried 4K7 resistor. But voltage spike is still there

Of course, we're not removing it just mitigating its effect. When power is turned off the 2v (or probably higher) pulse is dissipated through the catch diodes from ADS1115 input pin to supply rail. If the current isn't limited it can burn the input bonding. The 4k7 resistor limits the current to well under 10mA which the ADS1115 can handle indefinitely.

Incidentally, the output of the AD620 appears to rise to the common mode voltage when its powered off before dropping back to zero.

 

Of course, we're not removing it just mitigating its effect. When power is turned off the 2v (or probably higher) pulse is dissipated through the catch diodes from ADS1115 input pin to supply rail. If the current isn't limited it can burn the input bonding. The 4k7 resistor limits the current to well under 10mA which the ADS1115 can handle indefinitely.

Incidentally, the output of the AD620 appears to rise to the common mode voltage when its powered off before dropping back to zero.

Ok I got it.