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ADS1219: Recommended Amplifier for one channel

SndrS
Prodigy 10 points
Part Number: ADS1219
Other Parts Discussed in Thread: OPA320, ADS122C04, ADS1115, ADS131M02, OPA322

Hi all,

We am currently looking around how to improve a design to read lower voltages (amplify) from one of our sensors with a delta sigma ADC.

We are using the 24-bit ADS1219 with all four channels connected. One of these channels needs to read small voltage signals from a SiPM. We can, with the current gain of 4. 

However we would like to amplify this signal to at least 20x (25dB, gain). Voltage range: 0V-5V (supply can be -5V to +5V) and preferably low noise.

Can someone help me in the right direction finding a suitable OpAmp and/or reference design for this? Any other ADC recommendation with integrated Gain also welcome!

Thanks in advance! 

(actually this question might be more of a Amplifier question)

  • 0
    TI__Guru** 109595 points

    Hi SndrS,

    Welcome to the E2E forum!  There are a number of example amplifier circuits available here.  As far as op amps go I know the OPA320 has been paired with the ADS1219 in the past.  As you also mentioned your could also request information from the Amplifier forum.

    The ADS122C04 has an integrated PGA, but the problem here will be input range.  The PGA when enabled can't go all the way to the supply rails, and the input range narrows further when gain is applied.  This will be a typical issue when using a true PGA integrated into the ADC.

    The ADS1115 may be another option, but is only a 16-bit ADC.  However with this device the minimum full-scale range is +/-256mV, but can also measure up to 5V directly (with 5V VDD).

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hi Bob,

    Thanks for your quick reply and link. Thanks for 'OPA320' suggestion. I think adding an amplifier in our new design (just by footprint) would be a good idea.

    I just checked the "ADS122C04", a gain of 128 looks great (32x4), and double the speed (2SPS). But, why you think the input range is an issue (-2.85 to 2.85, 0 to 5.6)?

    The signal we receive from our sensor is pretty low. While the other 3 channels are 'only' used to read other signals, not so low/sensative..

    Would you advise to go for a dedicated single channel ADC with higher sample rate to read this single ended source? If so, what would you recommend?

    Well, maybe the ADS1115 might also be interesting, changing the a few bits (16vs24) to but a 256mV FSR.

    Best, Sander

  • 0 in reply to SndrS
    TI__Guru** 109595 points

    Hi Sander,

    The possible issue with the ADS122C04 is you have a limited input range due to the true PGA not able to drive the output to the rails.  See section 8.3.2 in the ADS122C04 datasheet on page 23.  The input voltage requirements are given in 8.3.2.1 in equations 6 and 7.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hi Bob,

    Thanks for pointing out so detailed (including page number). Much appreciated. Indeed, I did check these gains. But not sure if I follow this limited input range limitation.

    My expected sensor input range is expected to be between 0 and 5V (actually 0-4mV). Assume I want to use the 128 Gain, with a 0-5V (AVSS, AVDD) analog power supply. My maximum differential input voltage VINMAX=5V?, with a gain of 128 (FSR ±0.016 V) following the equation on page 24(7):

    "AVSS + 0.2 V + |VINMAX| · (Gain – 4) / 8 ≤ VAINP, VAINN ≤ AVDD – 0.2 V – |VINMAX| · (Gain – 4) / 8"

    Thanks, Sander

  • 0 in reply to SndrS
    TI__Guru** 109595 points

    Hi Sander,

    With the PGA enabled with the ADS122C04, even at a gain of 1 you cannot measure an input signal at AVSS (AGND) as your absolute input range is limited to within 200mV of the rail.  In other words if AINP is 4mV and AINN is at AVDD, you cannot measure this voltage as it is outside of the input range of the ADC regardless of whether or not it is inside the full-scale range.

    If the 0 to 4mV has a common-mode input of AVDD/2, then you could measure this voltage with PGA enabled and a gain of 128.  This is similar to a Wheatstone bridge measurement where the bridge is excited by AVDD and the output common-mode is at mid-supply. 

    So whether or not you can measure the voltage will depend on the absolute input voltage relative to AVSS.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hi Bob, 

    Thank you. Clear. That means, the ADS122C04 is indeed not a great option for my case. 

    Can you recommend an alternative one (maybe with a single channel) with a low FSR to read my single ended signal (16bit+)?

    Like the ADS131M02? (https://www.ti.com/lit/ds/symlink/ads131m02.pdf?ts=1696205896033)? Or would you then just advise to stick to the ADS1219 with maybe an extra amplifier (OPA322)?

    Best, Sander

  • +1 in reply to SndrS
    TI__Guru** 109595 points

    Hi Sander,

    You started out with I2C devices and now are looking at SPI?  The ADS131M02 has a limited AVDD supply (3.6V max) and a limited input range at gains higher than 4 (AVDD-1.8V) so I'm confused as to what you really want with your system and would suggest that you look at the parametric search for the specific types of measurement ranges you wish to make, interface types, PGA, resolution and supply ranges you wish to use.  Then check the specific datasheet for additional information as to reference voltage and input voltage range for the desired gain. 

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points

    Hi Bob,

    Yes, sorry for the confusion here. My question was maybe a bit broad. Actually I am looking for a way how to improve the accuracy of our prototype and seeking some recommendations. Indeed, we currently use the (i2c) ads1219. One idea was to boost the signal with an amplifier (initial thought), then maybe picking an Data Converter with a bigger gain can be an option. 

    Thanks for your help so far. We will go back to the drawing board. 
    Best, Sander