Continue to Site

# Back calculating the input from output signal chain with an isolated amplifier and opamp

Status
Not open for further replies.

#### newbie_hs

We are measuring the voltage of a charger using the below circuit with the help of an ADC present in a microcontroller.

The value of charger voltage is required for some other applications. The value of charger voltage is used to calculate isolation resistance.

I tried to calculate the input from the output. I am getting an error of almost 2 V in my calculations.

We are using an AMC1350 and a TLV6001 in the signal chain.

Please see my circuit diagram and calculations below.

From the circuit diagram you can see that the input V2 is 58 V. I used the equation shown in the second image to calculate V2 from Vout.

I am getting a value of 56 V instead of 58 V. Where did I go wrong?

Hi,

Why back?

I mean the simulation shows the voltage values.
So just verify them from the input.

58V --> R1, R2 --> it shows 2.294V, but I expect it somewhere around 2.377V.

You have a simulation tool, so use it.
Disconnect INP from the voltage divider R1, R2 and see what happens.
If the value now is correct, then the deviation is caused by the AMC,

If the value stays .. then most probably your 58V source includes some series resistance.

Klaus

BTW: it makes no sense to use an isolation amplifier when both sides refer to the same GND.
Mind to use all the capacitors as shown in the "typical application" schematic.

Last edited:
newbie_hs

### newbie_hs

Points: 2
I'm missing AMC1350 input resistance in your calculation. The respective gain error matches your results quite nicely.

KlausST

### newbie_hs

Points: 2
Hi,

Why back?

I mean the simulation shows the voltage values.
So just verify them from the input.

58V --> R1, R2 --> it shows 2.294V, but I expect it somewhere around 2.377V.

You have a simulation tool, so use it.
Disconnect INP from the voltage divider R1, R2 and see what happens.
If the value now is correct, then the deviation is caused by the AMC,

If the value stays .. then most probably your 58V source includes some series resistance.

Klaus

BTW: it makes no sense to use an isolation amplifier when both sides refer to the same GND.
Mind to use all the capacitors as shown in the "typical application" schematic.
Thank you..

I am desiging a isolation resistance measurement circuit. Using this document as reference.
In that document please see equations 13 and 14 for finding Riso.
For finding Vdc ,I am doing the back calculation.
The output of 2nd stage will be connected to an ADC

In real board GND's are different.
--- Updated ---

I'm missing AMC1350 input resistance in your calculation. The respective gain error matches your results quite nicely.
Thank you..
You mean gain error of AMC1350.

Rather gain error of your divider. You have 47k || 1.25M = 45.3k

Points: 2