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INA851: INA851 Driving Op Amp Application Note

Steve Kempf
Prodigy 60 points
Part Number: INA851
Other Parts Discussed in Thread: OPA1662, , OPA1622, PGA855, INA828, INA849

Hi, I was wondering if there are any application notes for using the INA851 differential output to drive an Op Amp input. I was planning to use an OPA1662 to amplify the INA51 output. The datasheet only lists application to drive a differential input ADC from the INA851 output. Appreciate any information. Thanks.

  • 0
    TI__Guru 54426 points

    HI Steve.

    The INA851 output stage function is similar to other fully-differential amplifiers, except the input and feedback resistors of the FDA stage are incorporated inside the device.  As you have mentioned, the device is fully-differential where both OUT- and OUT+ voltages are allowed to swing, centered around the VOCM output common-mode voltage.  The VOCM pin controls the output common-mode voltage.  For example, if VOCM is set to +2.5V, both VOUT- and VOUT+ produce a  fully-differential signal centered at +2.5V:

    The device can certainly be used to drive differential input instrumentation amplifiers input such as the PGA855, other fully-differential amplifiers, and/or a dual amplifier such as OPA1662 for a fully-differential system.  The input impedance of the OPA1662 in the inverting configuration is a function of the input resistors, therefore in general the INA851 will be able to drive it as long as the input impedance is around the ~2kΩ range.  I don't have a specific application note of the INA851 driving the OPA1622 but we can suggest circuit.  If you provide the input voltage amplitude, input common-mode, the required gain, bandwidth/frequency range of interest, and output voltage range and output common-mode requirements, and any other application specific requirements, we could provide suggestions.

    Thank you and Regards,

    Luis  

  • 0 in reply to Luis Chioye
    Prodigy 60 points

    Hi Luis,

    Thanks for your prompt reply to my post. I have the following Op-amp circuit parameters:

    Op amp: 1/2 of OPA1662

    Input voltage amplitude: 1 Vrms

    Input Common Mode: no specific requirement

    Gain: x4

    Bandwidth: 100 Hz to 16kHz

    Output Voltage Range 4 Vrms

    Output Common Mode: no specific requirement

    Thanks

  • 0 in reply to Steve Kempf
    TI__Guru 54426 points

    HI Steve,

    If you are using 1/2 of the OPA1662, does this mean you are only using a single op-amp? Is this a differential output solution or single-ended output solutions?  The INA849 and/or INA828 can meet the bandwidth requirement providing single-ended output at Gain of 4.  The input and output common-mode voltage needs to be within the linear range of the device.  Please clarify what sensor or signal you are amplifying.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    Prodigy 60 points

    Hi Luis,

    Yes using only a single Op amp in the 1662.

    Single ended output from the OPA1662 Op amp

    I am using the INA851 amplifier due to future needs to require differential output (this is a future requirement). The more immediate requirement is to drive an OPA1662.

    The input and output common mode voltage are in the range of the OPA1662. 1 Vrms in and 4 Vrms Out. The OPA1662 will have +15Vdc DC power supply and biased at half supply (+7.5Vdc).

    I am amplifying an electret microphone (250uV rated output) with the INA851.

  • 0 in reply to Steve Kempf
    TI__Guru 54426 points

    Hi Steve,

    In many electret capsule pre-amplifier reference designs, the op-amp is used on the transimpedance amplifier configuration to amplify the output current into voltage.  Below are a couple of designs for electret microphone using the pre-amplifier in the transimpedance amplifier configuration:

    https://www.ti.com/lit/an/sboa291/sboa291.pdf

    https://www.ti.com/lit/ug/tidu765/tidu765.pdf

    However, if this specific sensor is specified and rated to provide a ~250uVRMS signal at the INA851 inputs, the INA851 will provide the best noise performance when using the device in differential output configuration. One possibility, is to setup the INA851 on G=4000x, followed by the OPA1662 amplifier on a difference amplifier configuration with Gain 4x.  The OPA1662 in a difference amplifier configuration will convert the differential output signal of the INA851 into single-ended, providing best noise performance.  Please connect the INA851 thermal pad to the most negative supply V(-).  If using unipolar supply, V(-) = Thermal Pad = GND.

    Alternatively, if you wish to configure the INA851 in the single-ended output configuration, the OPA1662 could connect to one of the INA851 amplifier outputs.  For example, connect the OUT+ to one to the input of the OPA1662 on a gain configuration, while setting the VOCM pin to the mid-range of the output voltage swing at +7.5V, leaving the OUT(-) pin floating. The caveat of this approach is that the circuit will have additional noise since this configuration uses only half of the INA851 dynamic range, and this configuration will also exhibit additional offset and noise errors from the INA851 output common-mode voltage (VOCM) control amplifier. 

    Thank you and Regards,

    Luis