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PGA280

James Carter2
Prodigy 10 points
Other Parts Discussed in Thread: PGA280

We are prensently using the PGA280 in our design and we have a question regarding the noise of this component. We did a test by shorting the analog input pin to the ground and mesure the noise at the output of the PGA with a 24bits ADC. When we applied a gain between 1 to 16 the noise seems to be the same but when the gain is over 16 the noise begin to increase proportionaly to the gain. We want to know why the noise is so important with a gain of 128 and how we can improve our signal to noise ratio (for example between 64 and 128 the noise voltage double).

  • 0
    TI__Genius 12345 points

    Hi James,

    There are a couple of issues to keep in mind when reviewing the specifications for noise in a multi-stage amplifier such as an instrumentation amplifier or PGA. First, remember the noise spectral density numbers that are given are always input referred meaning they represent the noise spectral density of a discrete noise source if it was placed at the input of the amplifier. This is relevant because this number will be multiplied by the gain of the amplifier to determine the noise at the output. Second, in general two different noise sources need to be considered in the noise analysis: input stage noise and output stage noise. While the input stage noise WILL scale with gain, the output stage noise WILL NOT scale with gain and this leads us to an explanation of your first observation. The equation for the noise spectral density, referred to the output is:

    At low gains, the output stage noise will usually dominate, however as gain is increased, the input stage noise will dominate. This is why you are seeing relatively little change in output noise for gains less than 16. The noise at very high gains (e.g. 128) is almost entirely dominated by the input stage noise, hence why doubling the gain caused a proportionate increase in the output noise.

    Unfortunately we are bound by physics in terms of the amount of noise produced in the device and how it scales with gain. However one method to improve the signal to noise ratio would be to reduce the bandwidth of the system. The RMS noise voltage is the square root of the integration of the noise power spectral density over the bandwidth of the system. Therefore a 2X reduction in noise requires a 4X reduction in system bandwidth. This can be accomplished by filtering the output of the PGA or reducing the sample rate of the ADC if it has a mechanism for anti-aliasing.

  • 0 in reply to John Caldwell
    TI__Genius 12345 points

    I made a graph to illustrate the point I was trying to make in the previous post:

    The input referred noise spectral density, shown in red (EN RTI) will asymptotically approach the input stage noise value (approximately 22nV/rtHz on the PGA280) as the gain is increased, showing that the output stage noise is becoming less dominant. However, the output referred noise spectral density (shown in blue) shows the spectral density of the noise that will appear at the output of the amplifier when the effects of the amplifier's gain are included. So although the input referred noise is "decreased" by gain, there will still be more noise at the output.

    A good article related to this topic was published by TI's own Art Kay and can be found here: http://www.en-genius.net/includes/files/avt_032910.pdf