ADS7830: Reading full range

Nigel Morton

Part Number: ADS7830

Tool/software:

Hi,

I have an ADS730 with Vdd at 5.2V and a Vref of 5.0V (from an LP2980-5.0 to ensure stability).

I have my ADC input on CH1 and COM connected to Gnd for single ended input. The remaining ADC channels are also connected to inputs (so not floating).

I expected to read values from 0 to 255 for input voltages from 0 to 255 over the range from 0 to 5.0V but the ADC is returning the maximum 255 at 4.19V, well below the 5.0V of Vref.

I have set the reading with PD0 and PD1 set to 1 for Internal reference on and A/D converter on and I do not change these values.

I don´t understand why I am not getting the full range for the input.

Can anyone suggest what is wrong?

Thanks,

Nigel

over 1 year ago

0 Tom Hendrick over 1 year ago

TI__Guru**** 182721 points

Hi Nigel,

Welcome to our e2e forum! Can we start by reviewing your schematic connections around the ADS7830?

0 Nigel Morton over 1 year ago in reply to Tom Hendrick

Prodigy 10 points

Hi Tom, Here is my schematic. The input to oil_temp is a resistive load from 18-500 ohms (A VDO resistive temperature sensor)

0 Tom Hendrick over 1 year ago in reply to Nigel Morton

TI__Guru**** 182721 points

Thank you Nigel,

I'm not seeing anything jump out as being wrong - can I ask where you measured the 4.19V? Did you measure with an O'Scope or DMM?

0 Nigel Morton over 1 year ago in reply to Tom Hendrick

Prodigy 10 points

Hi,

I measured with a DMM between GND and pin 1.

I have built 2 separate boards and both are operating the same way so it appears to be device related and not a problem on a single board.

0 Tom Hendrick over 1 year ago in reply to Nigel Morton

TI__Guru**** 182721 points

Just to rule out a possible oscillation - can you look at the input with an o'scope?

0 Nigel Morton over 1 year ago in reply to Tom Hendrick

Prodigy 10 points

ok, just checked with a scope and no oscillation, absolutely steady.

0 Tom Hendrick over 1 year ago in reply to Nigel Morton

TI__Guru**** 182721 points

OK then - we have a few videos on the web describing how to drive the input and reference of a SAR converter. Please review these videos and try to verify that there is nothing influencing the reference and/or analog input.

https://www.ti.com/video/series/precision-labs/ti-precision-labs-analog-to-digital-converters-adcs.html?keyMatch=Driving%20a%20SAR%20ADC&tisearch=universal_search#

From there, expand the SAR ADC input driver (7) and Driving the reference input on a SAR ADC (6) topics., these should give insights on where to do further debugging.

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ADS7830: Reading full range