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PGA2500: Vin=2Vrms(2.88Vp)

ttd
Mastermind 8600 points
Part Number: PGA2500
Other Parts Discussed in Thread: PGA2505

Dear Technical Support Team,

I'd like to use Vin=2Vrms(2.88Vp) with PGA2500.
When the gain is 10dB, it means that ideal Vout is 2*10`(10/20) = 6.325Vrms.
Then the voltage of Vout+ and Vout- is

Vpp_dif=Vout_rms*2*root(2) = 6.325 * 2 * 1.414 = 17.890Vpp
 Vp_dif= 17.890/2 = 8.945V
 
Vp_dif= |Vout+ - Vout-|, so Vout+ or Vout- = +/-4.473V (VCM=0V).
 

Datasheet shows the range of Vout:　From（VA− +0.9）to（VA+ −0.9)
So Vout is +/-4.1V ( VA+=5V and VA-=-5V).
I guess that ideal output is very large with my condition, so it will be a large distortion.

Is it correct calculation?

I think that it is better for Vout to be under about 4Vrms, because following figure shows good THD+N under 4Vrms output.

Best Regards,
ttd

  • 0
    TI__Guru* 93105 points

    Hello ttd,

    Your calculations are correct, and your observation about keeping the differential output to about 3 Vrms to minimize the THD+N is correct. If you drop the PGA2500 gain down to the minimum of 9 dB (2.818 V/V), the 2 Vrms (2.828 Vpk) input results in differential Vout will be +/-4 V. That keeps the output swing way from the (V-) + 0.9 V and (V+) - 0.9V limits and the distortion should be low.

    Regards, Thomas

    Precision Amplifiers Applications Engineering 

  • 0 in reply to Thomas Kuehl
    Mastermind 8600 points

    Hi Thomas,

    Thank you for your reply.

    I understand that 9dB is good gain to input  2 Vrms (2.828 Vpk).

    By the way, is it possible to set 9dB with Gain amp of PGA2500.

    -------------------------

    The programmable gain amplifier can be

    programmed to either pass through the signal at unity gain,

    or apply 10dB to 65dB of gain to the input signal. The gain

    of the amplifier is adjustable over the full 10dB to 65dB range in 1dB steps.

    -------------------------

    Best Regards,

    ttd

  • 0 in reply to ttd
    TI__Guru* 93105 points

    Hi ttd,

    I errored. I had the PGA2500 and PGA2505 datasheets open at the same time and was looking at the PGA2505. The minimum gain setting of the PGA2500 is 10 dB, not 9 dB. Then, the only way to keep the PGA2500 distortion low is to reduce the input level so that the output swing stays within the low THD+ N range shown in datasheet graph.

    Regards, Thomas

    Precision Amplifiers Applications Engineering