Post Go back to editing

AD8231 vs AD8421 Selection

Category: Hardware
Product Number: AD8231

Dear Support,

I am in the design stage to choose an instrumentation amplifier from ADI to amplify a signal before 5V ADC(ADC121s051).  The instrumentation amplifier input signal has following parameters:

1. The signal has a frequency bandwidth from 100Hz to 150KHz.

2. the signal range is between 30uV ~ 30mV(peak-to-peak).

I will use 5V supply for the instrumentation amplifier and consider using the maximum gain of 128 from the AD8231 to get the maximum signal swing for the ADC input.

I have both AD8231 and AD8421 in my selection candidate list. I prefer to use the AD8231 since its wider frequency bandwidth, rail-to-rail output advantages and direct digital gain control. 

I would like to know:

a. what are the primary difference between the AD8231 and AD8421?

b. Whether it is a better choice to use AD8231 over the AD8421 for my application, or if I may have some things I miss if I choose the AD8231?

Thank you,

Lih

Parents
  • Hi  ,

    Good day. The main difference between AD8231 and AD8421 is the gain setting: AD8231 is a programmable gain instrumentation amplifier with software-programmable gains of 1, 2, 4, 8, 16, 32, 64, or 128 while AD8421 is an instrumentation amplifier with external gain settings. 

    The table shows the differences between the specifications of the two in-amp. Highlighted in green are the parameters with better specifications. 

    Overall, the AD8421 has a better bandwidth, noise, and slew rate but AD8231 has a lower offset which could be a critical parameter for your design. Looking into your design criteria, both parts can satisfy the bandwidth, but AD8231 excels in terms of the offset parameters. Hence, it is a better choice for your application.

    Can you share with us some other details of your application such as the Vocm and Vref to be use in your configuration? Thank you.

    Regards,
    Gilbeys

  • Hi Gilbeys,

    Thank you for your details in comparison between the AD8421 and AD8231.  As you recommended, if I choose AD8231 in my application, I will consider using the 2.5V as the AD8231 input bias voltage after AC coupling at the input, as well as for the output VREF as well.  

    ADI usually recommends that, since an instrumentation amplifier has a relative high output impedance, placing an operational amplifier between the instrumentation amplifier and an ADC will provide a low output impedance to interface with the ADC.  Since my application uses the ADI ADC121s051, is the operational amplifier in the AD8231 can serve this purpose?

    Thank you,

    Li0

  • Hi LiO,

    AD8231's opamp can be used as a buffer op-amp after the AD8231 InAmp. You can use LTSpice and simulate your front-end circuit with the AD8231 spice model - LTspice Download.

    Regards,

    Kristine

Reply Children
No Data