The measured value of AD7606 is about 4 times the actual value

Hello,

We would like to use the AD7606 for voltage measurements (only use 4 channels, no oversampling, parallel interface mode), and control it by FPGA.

First time, I forgot to decouple so there is an interference on CONVST A/B, leading to wrong throughput rate. But I got the correct voltage values, so I think there is no problem in my ADC transfer program.

I modified my circuit so this time I get the correct sampling frequency(50k). And I use oscilloscope to confirm that BUSY(0), CS(1), RD(2), CONVST A/B(3), FRSTDATA(4), RESET(5), CLK(6), OS0(7), OS1(8), OS2(9) are correct.

 

But this time I got wrong voltage values. When the value is positive, the measured value is about 4.5 times the actual value. When the value is negative, the measured value is about 4 times the actual value.

For example, square wave with 1Vpp, I got -2.06 to 2.19. Square wave with 0.2Vpp+0.2Vdc, I got 0.47 to 1.34; Square wave with 0.2Vpp-0.4Vdc, I got -1.98 to -1.16.

I have no idea how to solve it, maybe there is another mistake in my circuit design. I need some advice, thanks a lot.

 

Other chip resistors are all 100Ω.

Thank you for your time.



added tags
[edited by: Lluis at 9:25 AM (GMT 0) on 3 Oct 2019]