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ADS1158: ADC PN: ADS1158 Data sheet has conflicting data - What is the proper Vreference ??

ZVI PESHKESS
Prodigy 50 points
Part Number: ADS1158
Other Parts Discussed in Thread: REF3125, , OPA350

Hi:

 

We are using the ADS1158IRTC  with REF5040IDGK  as 4.096V  reference.

I have attached schematics.

Looking at the data sheet (https://www.ti.com/lit/ds/symlink/ads1158.pdf)

FIG-54  Page 42 in datasheet:

The reference pn:  ref5040  ( 4.096V)

however in the text above it mentioned REF3125 ( 2.5V). ""The 2.5V reference, REF3125, is filtered and buffered to provide a low-noisereference input to the ADC. The chop  feature of the ADC can be used to reduce offset and drift of the amplifiers." 

I think you have an error in the datasheet the reference shoud be REF3125  not REF 5040 ???

Is it legal to use the 4.096 Vref  as shown in our  schematics ?

per datasheet MAX value  for VrefP= AVDD +0.1V

                     MIN value  for VrefN=AVSS-0.1V.

We are getting serious errors when reading the ADC values.

please advise urgently.

 sincerel

zvi peshkess

zvip@marvintest.com

Link to documents

ADS1158

  • 0
    TI__Mastermind 44781 points

    Hi Zvi,

    First, the link to your schematic did not work. Unfortunately we cannot access Google Drive documents due to internal restrictions. If this is a PDF, can you just attach it to your next post and I can review?

    As you noted, the ADS1158 VREF voltage can extend up to the supply (AVDD - AVSS = 5V), so either the REF3125 or the REF5040 could work for the ADS1158 in Figure 54. My guess would be that either the image or text was updated at some point, causing a conflict between the two.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 50 points

    I have attached the PDF schematics page of the ADC.

    Please note that we are using +2.5V / - 2.5V for the power.The reference is 4.096V with regards to -AVDD.

    We are getting starange / not correct values.

    I think if we use 4.096V as reference the we are violating the terms:

    VOLTAGE REFERENCE INPUT Reference input voltage (VREF = VREFP – VREFN) 0.5 4.096  AVDD – AVSS V

    Negative reference input (VREFN) AVSS – 0.1V VREFP – 0.5 V

    Positive reference input (VREFP) VREFN + 0.5 AVDD + 0.1V V .

    what is the proper Vref source for our implementation ?

    see attached PDF.

    ADS1158 Schematics 3-10-2022.pdf

  • 0 in reply to ZVI PESHKESS
    TI__Mastermind 44781 points

    Hi ZVI,

    Since the REF5040 output voltage is with respect to the VREF ground, and you have the VREF ground = -2.5V, the 4.096V is relative to -2.5V (which is AVSS). This is almost exactly what is shown in the ADS1158 applications section (Figure 54), though you do not have the reference buffer (OPA350) which might be required. The ADS1158 VREF input impedance is only 40kohm, so adding a buffer between the REF5040 and the ADS1158 might be a good idea.

    What is going on with your measurements? Can you describe how they are incorrect?

    -Bryan

  • 0 in reply to Bryan Lizon86
    Prodigy 50 points

    will have ADC values Friday morning.

    In the mean time - can you verify that using the 4.098Vref in our configuration is correct ?

    thanks.

  • 0 in reply to ZVI PESHKESS
    TI__Mastermind 44781 points

    Hi ZVI,

    As per my previous post, yes, the way you have the REF5040 setup is correct. You just might need a buffer between the REF and the ADC. This is shown in Figure 54 in the ADS1158 datasheet.

    -Bryan