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OPA2313: How to handle the noise in small signal amplifier design

LLouis
Expert 3515 points
Part Number: OPA2313
Other Parts Discussed in Thread: OPA847, OPA388

Hi Team,

I am design a small signal amplifier that input down to 1uV and amplifier it to MCU ADC sampling. (Input range from 1uV to 100mV, low frequency signal)

After the prevision LAB learning, i calculate the noise possible introduced as below.  We use the OPA2313 as voltage follower in first stage. 

1. In this voltage follower design, as gain is #1, when calculating bandwidth, i use the GBP bandwidth 1MHz.

2. I calculate the noise to 32uV; 

////*** Process ***////

•140nv/SHz @ 1/f noise
•26nv/SHz @ Broadband
•BW = 1.57*1=1.57MHz
•Enbb=ebb*SBW=26*1253=32578nVrms
•Enormal=140*1=140nV
•Enflicker=140*4.07=570nV
•Etotal= 32582nV=32.582uV
////*** Process ***////

Now the questions coming:

1. Am i right in calculation?

2. If i choose the OPA847, a much low noise amplifier, BUT it have much higher bandwidth, which would introduce more noise. What i should handle in this case?

Thanks!

  • 0
    TI__Genius 15410 points

    Hi LLouis,

    To verify your calculations I recommend downloading the Analog Engineer's Calculator. In the tool there is a calculator to calculate the total noise of a circuit.

    Since your input voltage can be as low as 1uV you will require a much higher precision device than the OPA2313. The typical offset voltage of the OPA2313 is 500uV (500 times larger than your input signal). This means that the offset voltage of the OPA2313 will dominate your signal. I recommend reviewing the TI Precision Lab videos on Offset Voltage and Input Bias Current for more information.

    If you must use the OPA847 to meet your noise performance but you don't need the full bandwidth you can add an RC filter on the output of the amplifier to reduce the bandwidth of the circuit.

    Another amplifier to use would be the OPA388. This has much lower offset and is a low noise device.

    Thank you,

    Tim Claycomb