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OPA2365: Output DC bias shift by following AC input voltage

Part Number: OPA2365

Hi Guys

Our customer is using our opa2365 as AC input sample and condition circuit. Please find the schematic:

We writed some formulas to calculate the output voltage value. At the next test, we only focus on the DC value.

Bias voltage circuit: (We need the bias to lift up the AC sample voltage because the DSP/MCU only support positive voltage.)

You know at the OPA2365 AC input port. The AC range is 85VAC-265VAC for PFC input. The R2153 and R2145 is the same value 13.7k ohm. We just test the DET_FeedA_3  point DC Voltage(not DC+AC voltage, we can use Digital voltmeter DC gear)。In my opinion, ideally, the tested result should be equal to the Bias voltage. right? But we found the tested result will increase by following the increasing input AC voltage.

Vin(V)

DET_FeedA_3(V)  DC value

230

1.2501

240

1.2502

250

1.2503

255

1.2505

260

1.2520

265

1.2536

You can see the DC value increase by following the increasing input AC voltage. 

Action: 

I asked our customer to test the output voltage at different R2153 and R2145. Please find the new result:

You can see the Vdet and the R data: when the divided resistor is below 13K ohm, the tested voltage will keep stable 1.2462v or 1.2464v whatever the Input voltage value. But when the divide resistor goes up to 13.7k ohm. The output Vdet DC value will go up from 253V to 264V(AC). with the 15k ohm the point going up will be 232VAC. With the 16k ohm, the point will be 217VAC.  At the beginning of going up, we tested the DET AC peak value is 2.46 which is below the power rail voltage 3.3V, Right?

The right hand picture is the DET AC real waveform. we tested it. But almost no change during the whole test.

So question:

1. Why does the DC value of the DET voltage increase as the input AC voltage increases? 

2. Why is the beginning point is 13.7k? why does the lower resistor will keep the DET dc value table? such as 11k,12k... How to explain it?

3. For our application, generally speaking,what value should we define for the R2153 and R2145? Is there any calculation formula to explain it?

4.I have simulated the application circuit by using our TINA model. But no simulation result matched with the real test. Is it related to CMRR? or Resistor tolerance? Do you have some formula to support this idea?

This case is very urgent. Your feedback is very appreciated.

Thanks

-Pengfei