SBOA356 August   2020 OPA1656 , OPA210 , OPA2210 , OPA2320 , OPA2320-Q1 , OPA320 , OPA320-Q1

 

  1.   Trademarks
  2. 1 Introduction
  3. 2 Voltage Offset
  4. 3 Bandwidth
  5. 4 Cascaded Amplifier Bandwidth
  6. 5 Cascaded Amplifier Offset
  7. 6 Multi-Stage Amplifiers
  8. 7 Normal Distributions in Offset Voltage
  9. 8 Noise Considerations
  10. 9 Summary
  11. 10Resources
    1. 10.1 TI Recommended Parts
    2. 10.2 TI Precision Labs Training Videos
    3. 10.3 TI Recommended Resources

8 Noise Considerations

Precision amplifiers not only provide low offset, but also very low noise. There are three dominant noise sources to consider in amplifier design. These are voltage noise density, current noise density, and the thermal noise of the resistors in the circuit. The voltage and current noise densities are intrinsic to the op amp and the values can be found in the Electrical Characteristics section of the datasheet.

It is important that the thermal noise of the resistors do not exceed the intrinsic noise of the op amp. It would be a shame to pay extra for a low noise precision amplifier, just to allow resistor noise to dominate the system. Using smaller resistor values will prevent this from occurring. Lower value resistors will also limit the contribution of the current noise density. Specifically, source resistance at the non-inverting input and the equivalent resistance of the feedback network Rf|| Ri (Req) must be limited. For reference, Req for all the circuits in this document is ~82Ω.

It is worth noting that large resistor values can also interact with parasitic capacitance in the circuit to produce unwanted poles in the frequency response. These unintentional poles can lead to stability issues and should be avoided.

Similar to offset voltage, voltage noise can be modeled as a voltage source in series with the non-inverting input of the amplifier. This noise will be amplified by the non-inverting gain of the amplifier. The same methods used to limit Voso can be used to limit the output voltage noise of the amplifier.

Table 8-1 shows the input-referred noise of the various two-stage circuit designs considered in the Monte Carlo analysis. OPA2210 and OPA1656 have a broadband voltage noise density of 2.2nV/√Hz and 2.9nV/√Hz respectively.

Table 8-1 Output noise of a two-stage amplifier in four different gain implementations

Design 1

Design 2

Design 3

Design 4

GA1 (V/V)

OPA2210

200

50

31.6

10

GA2 (V/V)

OPA1656

5

20

31.6

100

1/f (0.1Hz to 10Hz)

(nVpp)

78.1

91.7

108.6

261.4

Broadband (1kHz)

(nV/√Hz)

2.4

2.4

2.4

2.5

Integrated

(0.1Hz to 91kHz)

(µVpp)

3.9

4.3

4.3

4.4

Total Integrated

(µVpp)

5.4

10.4

12

11.5

The broadband noise (noise above 1kHz) is very consistent for each design. This is because the low-noise OPA2210 was used as the first stage in all four circuits, and the contributions from thermal and current noise were greatly limited. Note that the super beta bipolar input transistors of the OPA2210 provide much lower 1/f noise compared to the FET input transistors of the OPA1656. Super beta transistors not only have lower 1/f noise, but lower bias current and bias current noise than traditional bipolar transistors.

Total noise is the integrated noise across the bandwidth of each amplifier. In this case, the broadband noise values are nearly identical, so the total noise becomes a function of bandwidth. That is, the higher the bandwidth of the circuit, the more noise is integrated as frequency increases.