AD7988-1, differential input, common mode rejection on VCOM

Hi

I have 0V to 5V signal on VCOM=2.5V where op-amp circuit (it active filter, gains, etc) sits between 0V and 5V. The signal is up to 100Hz B/W and it it not DC.

In case of VCOM wiggles, the signal would wiggles as common mode. The VCOM is driven by low impedance op-amp (+/-10mA drive).

I would like to attach these signal to the AD7988-1 which is 16 bits SAR (I'm looking into AD7732 later based on SD)

          IN- connect to VCOM (2.5V)

          IN+ connect to signal (OP2177 op-amp output) of +/-1.5Vpp (1V to 4V)

I connect AD7988-1:

          VREF = 4.096 (ADR364),

          VDD = 2.5V (ADR361)

and      VIO = 3V3 for SPI bus, sampling at 1KSPS on SPI bus. The AD7988-1 has internal clock for conversion processing.

My concern that

(1) The impedance of the IN+ and IN- is low but datasheet quoted that for high speed

(2) The impedance may be inbalance, not clear on datasheet

(3) The common mode rejection between +IN and -IN is not clear on datasheet

I'm trying to figure out what to expect in performance based on above circuit, the datasheet is rather limited in details.

Has anyone used this device based on above configuration based on low sample rate, is common mode rejection any good (before I get to instrument amp).

How to estimate actual impedance for lower sample rate, I can have 10nF cap between IN+ and IN- so it handle transient operation while sampling.

  • Hi Richard,

    Are you referring to the IN+ and IN- of the OP2177 or the AD7988? It is not indicated in the datasheet of the OP2177, but the input impedance of amplifiers are generally very high, typically in the range of Mohms or higher. The OP2177 also has a CMRR of 120dB minimum.

    If instead you are referring to the impedance and CMRR of the AD7988, I will request our administrator to move this thread to the Precision ADC community. I believe they will be able to help you with your concern.

    Thanks!

    Kris

  • I was referring to connection of VCOM to the -IN of the ADC along with VSIG to the +IN which come from the OP2177 device.

    I can set VREF to 4.096V using ADR43x, so we have signal range +/-2V on VCOM =2.0V.

    I wish to establish common mode performance of the ADC based on above.

    The OP2177 is used for x1 to x100 gain stage (digital pot).  It does not do common mode rejection.

    The sampling rate is 1KSPS. I also looking into AD7732 which is sigma delta. I trying to figure out pro and con, the only con I can see that AD7732 need external XTAL.

    I will attached picture this evening, the internet here is pretty bad.

    R.

  • Attached image

    NB: The ROUT and COUT subject to review depending on drive to IN+ and IN-. Should each COUT tied to RCVGND for both +IN and -IN?

    The signal size is +/-2V on 2V0COM (virtual common) using TI op-amp (anycap series)

    Regards

  • Hi,

    To answer your initial questions on the AD7988-1:

    (1) The impedance of the IN+ and IN- is low but datasheet quoted that for high speed and (2) The impedance may be inbalance, not clear on datasheet

    • The AD7988-1 has a dynamic impedance by virtue of its architecture. Please see Figure 32 on page 16 of the datasheet. In the acquisition phase, the impedance of the analog inputs (IN+ and IN−) can be modeled as a parallel combination of Capacitor CPIN and the network formed by the series connection of RIN and CIN. During conversion, input impedance is limited to CPIN.

    (3) The common mode rejection between +IN and -IN is not clear on datasheet

    • The analog input CMRR is under the analog input specifications. For an input of 1kHz, it is 60 dB.

    I am curious as to why you opted for the two-channel version of the OPx177 family. What do you use the other channel for? We do have a Circuits from the Lab reference circuit for AD7988-5. You may check CN0305 (http://www.analog.com/en/circuits-from-the-lab/CN0305/vc.html) for reference and to see the system performance.

    Hope this helps.

    Regards,

    Karen

  • Hi Karen

    Thank for the reply

    (1) The common mode rejection remains unclear when used with AIN- to 2V0COM and AIN+ to signal input. It is not quoted in the datasheet under this configuration. This apply to specific setup that I cannot use.  

    (2) The CN0305 is based on AIN- connected to AGND, rather than 2V0COM. Also the PSU is different where I only operate to 0V (AGND) to 5V. My question is this: would the system performance in CN0305 actually apply to the circuit I was trying to implement, if not, what would be a significant change?

    (3) The schematic is only snapshoot of the circuit I used, there are several op-amp prior to this circuit.

    I trying to affirm confidence in using AD7788-1 based on schematic I provided, I see no example or performance parameter in the datasheet since they use different setup. 

    I was wondering what the pro and con between AD7732 (SD) and AD7788-1 (SAR)? The sample rate is 1KSPS. One pro of AD7732 that it has buffer and high impedance input. The con of AD7732 that it specified to 105C (not 125C).

    Regards

    R.