the output from AD620 is giving output less than expected (Almost 60% of expected value). What can be the issue?

hi ano,
Has this simple test explained the reason for the original problem?
E

 

I had noticed a small offset in the In-Amp output with only single input pin connected to GND. I will try connecting both input pins to GND through 100k resistors. I expect this will eliminate offset error completely.

Only if the offset is from input bias current and not the intrinsic input voltage offset.

 

Only if the offset is from input bias current and not the intrinsic input voltage offset.

OK. noted

 

I was replying to his specific test using the millivolt source input, not for a general application.

I was addressing the application shown in post #1. That was boosting the output from an actual bridge.

 

This is a amplifier circuit for Wheatstone bridge output. Bridge is excited by constant current.
Is it OK to take GND from centre tap of transformer? Whether this will results in noise in In-Amp or ADC outputs?
(For simplicity, I have not shown filter caps and other safety components in the schematic)

 

For this circuit to perform in a reasonable manner it is required that the center tap of the TRANSFORMER BE TIED TO THE CIRCUIT COMMON, WHICH MAY ALSO BE GROUND.

 

Hi,
The TS's circuit clearly shows the Gnd symbol on the transformer CT and the Logic grounding/common, they are connected.

E

 

I was addressing the application shown in post #1. That was boosting the output from an actual bridge.

I was well aware of that.

 

OK. noted

Offset doesn't reduced when I pull down both AD620 input. As you mentioned this can be intrinsic error of millivolt source.

 

Offset doesn't reduced when I pull down both AD620 input

Hi anoo,
What does the Vout measure when the inputs are both 0V low?
E

 

Hi anoo,
What does the Vout measure when the inputs are both 0V low?
E

Millivolt source / AD620 OUT
0.000 / 1.34
5.000 / 6.34
10.000 / 11.34
20.000 / 21.38

 

Hi anoo,
It is very important you state the Units when posting a value, is it Volts or mVolts???
Also what is the 0.000 thru 20.00 measurement?


Is it 5.000mVin gives 6.34mV ??
E

 

Hi anoo,
It is very important you state the Units when posting a value, is it Volts or mVolts???
Also what is the 0.000 thru 20.00 measurement?


Is it 5.000mVin gives 6.34mV ??
E

Oh. I missed to mention the units. All values are in millivolt. All AD620 indicated values are 1.34 mV higher than indicated in millivolt source.
Please note that AD620 amplified the millivolt signal [Gain=50.4] and its output is fed to ADS1115. What I have indicated is the measurement of ADS1115 which is operated with unity gain [So measurement= COUNTS X LSB ÷ AD620GAIN]

 

Hi,
The d/s for the AD620 shows a possible maximum offset of 1.000mV at +/-15v.
How is the mV input source generated? And also how is it connected?

Do you have a photo to post of the project?

E

 

Hi,
The d/s for the AD620 shows a possible maximum offset of 1.000mV at +/-15v.
How is the mV input source generated? And also how is it connected?

Do you have a photo to post of the project?

E
View attachment 332476

Millivolt source is Yokogava CA100

 

Hi aoo,
A different point, I do not see any capacitive decoupling on the Voltage regulators.???

E

 

Hi aoo,
A different point, I do not see any capacitive decoupling on the Voltage regulators.???

E

Yes I purposefully dropped them. This was the first circuit I made by myself (even I soldered first time) so the confidence level that this circuit works was too low so I neglected those caps. As I used switching supplies for ad620, wiring is messy, filter caps are absent, Gaussian noise at output was high. I managed it through averaging at microcontroller level. Presently I am planning to improve the circuit.
But I presumes filtering has no relation with offset error.

 

Hi, aoo,
Without good decoupling at these low signal levels will get you problems.

For the mVout offset problem, disconnect R7 from pin #4 of U2 and connect it to 0v/gnd.
Measure Vout again, and post the result for 0mV input to the AD620
E

 

How do you provide a millivolt output from the Multimeter?
It must have a DC path to ground for the the amp input bias currents, and not be floating.

I got a schematic of a readymade AD620 module available for purchase. Whether this circuit works if S+ & s- is connected to a standalone millivolt source. I think this circuit also face same issue related to input bias currents.