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PGA309: PGA309 bandwidth is inconsistent with datasheet

Kailyn Chen
Genius 9560 points
Part Number: PGA309

Hello，

One customer meets the bandwidth question about PGA309.

1) .The signal generator output sinewave with +-20mV, adding 2.5V bias to PGA309 input to meet the input common voltage requirement.

2) The front PGA Gain=32, Gain DAC=0.5, output amplifier GAIN=2.

2) Changing the frequency of sinewave, from 20Hz~200Hz, record the frequency and output amplitude.

He found that the bandwidth is 165Hz, while the datasheet described that the front-PGA bandwidth is 60Khz ,and the output amplifier bandwidth is 2Mhz.

Because there is no PGA309 spice model for simulation, so I need your advice. 

Thank you very much for your help.

Best regards

Kailyn

  • 0
    TI__Genius 10295 points

    Hi Kailyn,

    Can you provide a schematic of your PGA309 circuit? Do you have any input or output RC filters that could be attenuating the signal?

    How are you measuring the bandwidth? Can you provide scope captures of the input and output waveforms?

    Regards,

    Zach

  • 0 in reply to Zach Olson
    TI__Genius 9560 points

    Hi Zach,

    Thank you very much for your reply, the information are as following:

    And the tested -3dB result is about 165Khz.

    Best regards

    kailyn

  • 0 in reply to Kailyn Chen
    TI__Genius 10295 points

    Hi Kailyn,

    Thank you for providing the schematic and captured waveforms.

    The bandwidth of your circuit is limited to ~165Hz due to the large (47nF) capacitor placed between Vsj and Vout. This capacitor (C3) is in parallel with the gain-setting feedback resistor of the output amplifier which is ~18kΩ for gain = 2V/V. The cutoff frequency of this RC filter is calculated to be 188Hz. The PGA309 datasheet shows a more typical value of only 150pF for this capacitor which results in a cut-off frequency of just under 60kHz.

    Also note that you have an input low-pass filter with a cut-off frequency of ~3kHz, due to the 5kΩ input resistors (R4 and R5) and 10nF capacitors (C13 and C14).

    Regards,

    Zach