ADS122U04: ADS122U04：DynamicRange with Single- end input

Hi Shing,

See my responses below.

Best regards,

Bob B

Masa_0000 said:

Part Number: ADS122U04

HI everybody!

I have a question about the ADS122U04 datasheet.

Q1:what value about DynamicRange with Single- end input.

    my consideration is followings:

   -------VREF:2.048V      AIN2:GND    AIN: 2.048V max. ---0V min.   =Input Dynamic Range for Single- end

Refer this figure:

[Bob] The figures below show a pseudo-differential measurement (single-ended) and a true differential measurement.  In the case of a single-ended measurement, one input is held at a constant voltage.  Often this voltage is at analog ground, but it could be at some other voltage.  With a unipolar analog supply (+5V/AGND for example) the input voltage is not allowed to go below ground.  If the AINN input is tied to ground and the AINP input travels from 0 to 2.048V, then with a 2.048V reference the dynamic range is 1/2 (+ half) of the full-scale range (+/- 2.048V), or 23 bits.  This is more significant with the 16-bit device (ADS112U04), but not as significant with the ADS122U04.  This is because of noise.  With the 16-bit device all 16 bits are usable, whereas with the 24-bit device the noise reduces the effective resolution (ENOB) to less than 23 bits.

Another consideration is the ADS122U04 is considered a DC type sensor device in that its primary purpose is to measure signals near DC levels.  The ADS1292 (ADS1192) is considered an AC device.  So comparing devices is not an apples to apples type comparison.  You would need to also consider the data rates and filtering of the input signals.

Q2:Data sheet 10 pages     From the description,"Pin Functions"

REFP REFN is written as an analog input.
When not using it, is it OK to connect to AVDD?

[Bob]  The REFP and REFN inputs are analog differential input pins.  These pins can be connected the same as any other analog input.  The only restriction is based on the Absolute Maximum Ratings for the Analog input voltage as specified in table 6.1 of the datasheet.

"9.1.5 Unused Inputs and Outputs"
As described in Is there a problem with connecting AVDD to REFN?

[Bob] If not being used you can connect the AVDD to REFN as this input is an analog input.  You can leave it floating or tie to AVSS as well.  If there is concern with a direct connection to AVDD, then use a pull-up.

Q3:"9.1.5 Unused Inputs and Outputs"

From this description of
It seems possible to use both internal reference and external reference, but are there any concerns?
With external connection, switch to the internal setting by register setting and use, there is no unnecessary flow?

[Bob] Yes, both internal and external references can be used in the system.  This is often the case for ratiometric measurements where a thermistor or RTD is used to measure the cold junction of a thermocouple input.  The excitation source establishes the external reference used in the cold junction measurement. The internal reference is also used to measure the thermocouple voltage.  There is a mux internal to the ADS122U04 that selects which reference source is being used and is selected in the Configuration 1 register.

If you have any information please let me know.

Best Regards

Shing