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PGA280: noise floor

Part Number: PGA280
Other Parts Discussed in Thread: PGA281, PGA103, PGA202, OPA2320

Hi team,

Our customer uses PGA280 + AD7765 for signal acquisition

Short the input of AD7765, the noise level is low. However, when PGA280 + AD7765 are tested together, the input is shorted, and the gain is 1. After the acquisition of AD7765, the noise level is very high, and the it is about 0.5mv. Check the ripple performance of the power supply, and it's OK. What is the possible reason? Is there any way to eliminate this noise floor? 

  • Hi Amy,

    The PGA280 is a programmable gain amplifier optimized for DC precision measurements and often used for relatively low frequency signals. It is often paired with delta sigma converters that use a relatively low output data rate, where signals in the low frequency range or DC measurements are of interest. Below is a total noise TINA simulation plot for the PGA281 with Gain =1 (the PGA281 uses same core amplifier as PGA280).  The expected total output noise of the PGA280, stand alone, on gain of 1V/V in the circuit configuration above is @ 1MHz is 136uVRMS (or ~0.9mVp-p).

    The total noise on the system will be also dependent on the delta-sigma data rate and digital filter settings of the delta sigma converter. For example, at output data rate of ~156kHz, the digital filter of AD7765 specifies a -3 dB bandwidth of 64kHz. At this frequency the noise of the PGA280 is expected to be around ~59.8uVRMS (or equivalently ~0.4mVp-p).  The noise you see at 0.5mVpp in the system appears to be close to the expected noise amplitude that is expected for this instrumentation amplifier on these settings.  One suggestion to reduce noise could be to slow down the data rate of the Delta-sigma or reduce the f(-3dB) bandwidth on the delta-sigma digital filter settings.

    If you see noise that grossly exceeds this, please let us know what is the amplitude and frequency of the ripple of the voltage supplies, as well as the devices used to generate all the PGA280 voltage supplies (+/-10V and +5V).

    Thank you and Regards,

    Luis

      

  • Hi Luis,

    Thanks for your work. It's very detailed.

    Our customer needs me to recommend a low-noise PGA that can be used with AD7765. There are the following requirements. I checked the PGA that meets the customer's requirements, and the noise performance is similar. Relatively speaking, PGA103 and PGA202 are better. Can I recommend it to customer? Do you have any good suggestions

    1、The bandwidth is between 10-10khz,

    2、Differential input is preferred, and single ended input is also acceptable

    3、power supply ± 10V 

    4、Gain 1, 2, 5, 10, 20, 100 are adjustable

  • Hello Amy,

    The PGA202 and PGA103 are the only programmable gain amplifiers that support the gains of 1, 10, 100 and the +/-10V power supplies.

    The PGA103 offers single ended input with noise density of 11nV/sqrt(Hz) and low frequency noise 0.6uVp-p from 0.1-10Hz.

    The  PGA202 offers the differential input, with noise density of 12nV/sqrt(Hz) and low frequency noise 1.7uVp-p from 0.1-10Hz.  The allowed input common mode for the PGA202 is 5V from the rail supplies.  

    Both of these PGA's offer single-ended output, so a dual amplifier stage such as the OPA2320 can be used to perform the single-ended to differential conversion and shifting of the signal to 2.048V common-mode as required by the AD7765 modulator. See a possible circuit below.

    Thank you and Regards,

    Luis