ADS8861: The Dout pin gives a constant high (i.e '1') as an output .

Haseeb Afzal Mir

Part Number: ADS8861

Tool/software:

I am using the FPGA get the sampled data. I am using a 3-wire operation and so far I have taken everything from the data sheeet .

Sclk :- 25 MHz (40 ns).

Time between conversions : Well beyond 1000ns.

Acquisition time :- 640 ns . (well above the limits of min acquition ).

Attached are the waveforms below .

Waveform 1: The bottom on is the convst signal . The middle one is the Sclk . and the one above it is Din (Constant high at 3.3v). Pink one is the input wave which is to be sampled.

Waveform 2:

Its shows the time between conversions are also good .

The out of the Dout is a constant 3.3V high waveform . Any solutions ?

9 months ago

+1 Samiha Sharif 9 months ago

TI__Expert 8756 points

Hi Haseeb,

Thanks for your question. To confirm, what is the voltage of the input signal applied? What VREF and AVDD and DVDD values are you using? What happens if you change the input signal value?

Best regards,

Samiha

0 Haseeb Afzal Mir 9 months ago in reply to Samiha Sharif

Prodigy 10 points

Hi Samiha .

Thanks for responding.

Input voltage (Differential voltage ) is a sinusoid voltage (peak = 5 ) . So I put voltage will vary from +5 to -5 V .

AVDD — 3.3V

DVDD — 3.3V

VREF — 5 V .

I am testing it with a 5V DC voltage and varying the DC from -5 to 5V . Dout is constantly 0 (previously it was a constant 1. We made some changes and it’s now giving constant 0 ) . It isn’t giving out any data

0 Samiha Sharif 9 months ago in reply to Haseeb Afzal Mir

TI__Expert 8756 points

Hi Haseeb,

Yes, if you set VREF to 5V and are applying a differential +/-5V input, you should be able to see it. Are your AINP and AINM inputs 0V to 5V, centered around 2.5V? The inputs cannot accept bipolar signals. Could you please share your schematic as well so I may review it?

Also are you trying to operate in 3-wire with BUSY or without BUSY mode?

Best regards,

Samiha

0 Haseeb Afzal Mir 9 months ago in reply to Samiha Sharif

Prodigy 10 points

The schematic is below

Layout has been followed as given below

Below are the AINP and AINM signals .

AINN and AINP are both varying from 0-5 V centered around 2.5V as shown below.

I am using it in a 3 wire mode without busy indicator.

+1 Samiha Sharif 9 months ago in reply to Haseeb Afzal Mir

TI__Expert 8756 points

Hi Haseeb,

Thanks for providing the screenshot. This is strange as everything looks alright. Your schematic looks correct. Could you try using the BUSY indicator and see if it toggles? Are you seeing this issue with only one device/one board? It may be worth swapping the device with a new one in case this is a one off issue.

Best regards,

Samiha

0 Haseeb Afzal Mir 9 months ago in reply to Samiha Sharif

Prodigy 10 points

Hi Samiha ,
I figured it out . Turns out my ground pin was not properly touching the pad .

I am getting a bias of 40 at 0V . That is acceptable or it can be further improved ?

0 Samiha Sharif 9 months ago in reply to Haseeb Afzal Mir

TI__Expert 8756 points

Hi Haseeb,

Very glad to hear! You may choose to calibrate the ADC in your system as described here: https://www.youtube.com/watch?v=zARSuyJtHYg

Best regards,

Samiha

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ADS8861: The Dout pin gives a constant high (i.e '1') as an output .