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Original question:

PGA113: PGA113

PGA113: PGA113

imtiaz khan
Intellectual 430 points
Part Number: PGA113
Other Parts Discussed in Thread: OPA325

Hi All

I am using PGA113 in unity gain configuration to drive a C8051F507 uC A/D input with a sampling capacitance of 32pF and input multiplexer resistance of 3K. Do I need to isolate the two with a resistance for stability or can the two be connected directly.

Regards

ik

  • 0
    TI__Mastermind 49445 points

    Hi IK,

    From the stability perspective, the device could drive directly the 3kohm multiplexer input resistance and the 32pF ADC input sampling capacitance ADC without oscillations or stability issues.   The PGA113 is characterized in the Electrical Characteristics table driving a parallel combination of a 10kOhm load resistor and 100pF capacitor.

    In general, when driving SAR ADCs, we recommend an external RC charge kickback filter (or charge bucket filter) at the input of the ADC, to help the amplifier charge the sample-and-hold capacitance and microcontroller mux capacitance during the ADC Acquisition time.  Using an RC filter with ~330pF with a 200-ohm resistor may help with the output step transient response of the PGA113 while charging  the ADC+Mux capacitive input.  Place the RC filter right at the inputs of the ADC (microcontroller).

    Thank you and Best Regards,

    Luis 

  • 0 in reply to Luis Chioye
    TI__Mastermind 49445 points

    HI IK,

    I wanted to mention, the PGA113 small-signal bandwidth changes depending on the gain setting.  For example at G=1V/V, the small signal bandwidth is ~10-MHz, while at G=200V/V, the bandwidth is only 230-kHz.  In general, the amplifier's closed loop output impedance is inversely proportional to its bandwidth, where amplifiers with higher bandwidth offer lower output impedance over frequency, and therefore can drive ADCs at faster sampling rates (settle in shorter acquisition periods).

    The minimum acquisition period for the C8051F507 uC ADC is 1.5us.  The PGA113 will be able to drive the ADC at max. throughput when using G=1-V/V, but will probably have issues settling within the  acquisition period when using the PGA113 at G=200V/V...  

    If you intend to drive the ADC at full-sampling rate of 200-kHz, using the PGA113 at a high gain setting, I recommend you buffer the PGA113 with a relatively high bandwidth amplifier such as the OPA325 (10-MHz bandwidth).  Alternatively, if a fast ADC sampling rate is not required, you may be able to program a longer acquisition time on the ADC and use the PGA113 without a buffer.

    Thank you and Regards,

    Luis