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TIDA-00176: Question regarding THS4531A amplifier gain settings

user5900199
Prodigy 50 points
Part Number: TIDA-00176
Other Parts Discussed in Thread: THS4531A

As described in the TIDA-00176 manual page 21 (chapter 4.1.1.3) the gain of the THS4531A amplifier is set to 5 for a 1Vpp input signal. This results in a 5Vpp differential signal at the output. Because we have a 2.5 common reference and 5V power supply the  outputs swings from 0-5V which should not happen.

When reading the following text:

Due to the gain of 5, a typical 1-V PP input signal leverages around 50% of the ADC full-scale range (FSR),
which results in a loss of 1-bit of precision, therefore yielding an equivalent 15-bit resolution. The lower
input voltage of 0.6 V PP will leverage around 25% of the FSR, which equals typically 14-bit resolution.

I assume that a gain of 2.5 would be the right choice. What I'm thinking wrong here? 

 

  • 0
    TI__Prodigy 640 points

    Hi,

    The op-amp THS4531A has rail to rail output capability and hence a 1Vpp input with 2.5V reference input will result in an output swing of 5Vpp or 0-5V. This is within the op-amp capability and hence there is no issue. However, worst case linear output range is 0.2- 4.75V and hence a slightly lower gain than 5 is advisable to enable full use of op-amp range as well as to stay within the linear region.

    Regards
    Siva

  • 0 in reply to Siva Mohan
    TI__Expert 3300 points

    Hello,

    please let me correct: A gain of 5 is perfectly okay with the THS4531A fully differential amplifier. Please note that we have differential signals and and we talk peak to peak voltage of differential signals, which are twice the peak amplitude.

    A Sin/Cos encoder has differential output signals with each signal (Sin+ and Sin- or Cos+ and Cos-) having a 0.25V peak amplitude only. Considering (Sin+) - (Sin-) we get a differential peak amplitude of 0.5V, which is 1V peak to peak. 

    Now let's amplify these differential signals by a gain of 5. Each differential signal e.g. Sin+ and Sin- now have an amplitude of 1.25V and with a DC offset of 2.5V they are very well within the linear range (1.25V to 3.75V) of the THS4531A.

    How do we get 5 Vpp: The differential peak amplitude between Sin+ and Sin- is only 2.5V. Since it swings between +2.5V and -2.5V the peak to peak voltage is 5 Vpp. 

    Therefore a gain of 5 is perfectly okay with the THS4531A fully differential amplifier. 

    As explained this gain even only leverage around 50% of the THS4531A output range assuming 1Vpp differential encoder signals, but this is recommenced to have additional margin, since 1Vpp is the nominal output voltage of encoders, but the maximum can be e.g.1.2 Vpp or even higher pending vendor. 

    Regards,
    Martin