AD7738 voltage shift?

[asrock70]

AD7738 is 24bit 8 input ADC with AIN, AINCOM Common-Mode Voltage from AGND+0.2V to AVDD – 0.3 V.
How convert (shift) 0-2.5V?

 

[FvM]

You have to use an external instrumentation amplifier. Or operate the AD7738 in unbuffered mode, which allows an input voltage down to 0.

 

[asrock70]

Analog Device in datasheet write: "Only operation with internal buffer enabled is recommended."
Question is must use one amplifier for every AIN or can use for example
Set AINCOM on 1,25V , on AIN0 set measured voltage 0-2,5V and betweent MUXOut+ and ADCIN+ integrate Amplifier shifting voltage about 1,25 i.e. from 0 - 2,5V on 1,25-3,75V?

 

[FvM]

Personally I would prefer a solution that keeps the input zero and allows a zero system calibration. AINCOM can be be used as an input GND reference, the shift has to be applied between differential MUXOUT and ADCIN. Basically you need an instrumentation amplifier with a common mode range down to GND.

 

[asrock70]

I'm a bit dopey at times, can give you an example?

AINCOM=GND, AIN0=0-2.5V i.e
MUXOUT(+)=AIN0=0-2.5V
MUXOUT(-)=AINCOM=GND
....

 

[FvM]

My suggestion was to connect a single supply instrumentation amplifier with input range down to zero, e.g. AD623, between MUXOUT and ADCIN. REF/ADCIN- would be connected to reference voltage within the ADC input voltage range.

 

[asrock70]

Certainly, on REF input I forgot.
I thing idealy IA for this is AD8295 , integrated A2 with R1/R2 solve divider and buffer for divide 1,25V from 2,5V reference.
Unfortunately , in our country is not accessible (Farnell,etc. big zero ) :|
Thanks.

 

[FvM]

A different approch would be to operate the ADC with a dual supply (e.g. -0.5/3.0V) and apply a level shift in the digital interface. Some ADC manufacturers (e.g. Cyrrus) have introduced an auxilary negative supply for the input buffer to overcome the common mode range problem of single supply devices.

 

[asrock70]

Already now have in device few voltage level and some part float.
I use IA ,try AD8221, currently search inexpensive low noise voltage source for shifting (0.5-2V).
I test resistor divider 10k/10k from ADR431 and OP727 as buffer, but voltage ripple on REF input not descent 150uV p-p.
ADR430 is not accesable (Farnell only in USA stock and price not very good )

 

[FvM]

As I mentioned before, I would keep the input zero across the shift operation, so the shift amount only appears as a common mode voltage to the ADC and thus isn't involved in calibration. Of course, the IA still contributes to measurement error, both offset and scale. Offset can be eliminated by a system calibration, possibly also scale, depending on the design.

But I agree, that the restricted common mode range of single supply ADCs is always somewhat annoying. I don't see a general solution to it, the trade-off has to be found for each individual design a new.