SBOA551 June   2022 INA240

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2One, Versus Two Reference Pins
  5. 3Bidirectional Current Sense Amplifier Topologies
    1. 3.1 Single-Stage Difference Amplifier
    2. 3.2 Difference Amplifier Input Followed by Noninverting Output Buffer
    3. 3.3 Voltage Feedback Multi-Stage Difference Amplifier
    4. 3.4 Single-Stage Current Feedback
    5. 3.5 Current Feedback Multi-Stage Difference Amplifier
    6. 3.6 Isolated Bidirectional Current Sensors
  6. 4Options for Driving Reference Pins and Input Referred Reference Error
  7. 5Resistor Divider as Reference
    1. 5.1 Resistor Divider and Equivalent Circuit
    2. 5.2 Reference Source Impedance Error in Difference Amplifier
    3. 5.3 Reference Source Impedance Error in Voltage Feedback Multi-Stage CSA
    4. 5.4 Reference Source Impedance Error in Current Feedback Multi-Stage CSA
    5. 5.5 Reference Source Impedance Error in Difference Amplifier with Output Buffer
  8. 6Examples
    1. 6.1 Calculating Reference Source Impedance Error in Difference Amplifier
    2. 6.2 Calculating Reference Source Impedance Error in Voltage Feedback Multi-Stage CSA
    3. 6.3 Calculating Reference Source Impedance Error in Current Feedback Multi-Stage CSA
  9. 7Summary

6.2 Calculating Reference Source Impedance Error in Voltage Feedback Multi-Stage CSA

Table 6-2 Reference Source Impedance Error for INA240A2
Rx (kΩ) C = Rx / (Ri_2 + Rf_2) m = C / (1 + C) Err_Vref (mV) Err_Vcm (mV) Err_Vdiff (mV) Err_Total (mV)
1 0.0057 0.0057 –5.68 14.20 2.84 11.36
5 0.0286 0.0278 –27.78 69.44 27.03 55.56
10 0.0571 0.0541 –54.05 135.14 13.89 108.11
20 0.1143 0.1026 –102.56 256.41 51.28 205.13
30 0.1714 0.1463 –146.34 365.85 73.17 292.68
40 0.2286 0.1860 –186.05 465.12 93.02 372.09
50 0.2857 0.2222 –222.22 555.56 111.11 444.44

Referring to Figure 5-4 output stage, and using INA240A2 as an example, the design values for input (Ri_2) and feedback (Rf_2) resistors are 50 kΩ and 125 kΩ respectively. Use these nominal values in the calculations shown in Table 6-2. It is known that the common-mode voltage is set at half supply.

Table 6-2 is under the condition of: Vs = 5 V; Vref_x = 1 V; Vdiff = 50 mV.

The common-mode voltage of the output stage is set at half-supply and is independent of the potentially high input common-mode voltage seen at the input of the device. Therefore, the output error term is decoupled from the common-mode voltage of the device. As a result, if the reference voltage is also set at half supply, the two error terms cancel, regardless of the source impedance. However, the term due to differential input still remains. In this table, a 50-mV input to the device input is used as an example, because the front stage is in a gain of 20, the output stage sees and effective differential input of 20 × 50 mV = 1 V.

The second example is INAx191 and INA186. The INA186A2 is used to illustrate. Although the input stage is different from that of INA240, the output stage is very similar. The design values for input (Ri_2) and feedback (Rf_2) resistors are 400 kΩ and 1 MΩ respectively. The common-mode voltage is set at one-third of supply. Again, the nominal values are used in the following calculation.

This information can be used to generate Table 6-3 for the output error terms, under the condition of: Vs = 5 V; Vref_x = 2.5 V; Vdiff = 50 mV.

Table 6-3 Reference Source Impedance Error for INA186A2
Rx (kΩ) C = Rx / (Ri_2 + Rf_2) m = C / (1 + C) Err_Vref (mV) Err_Vcm (mV) Err_Vdiff (mV) Err_Total (mV)
1 0.0007 0.0007 –1.78 1.19 0.36 –0.24
5 0.0036 0.0036 –8.90 5.93 1.78 –1.19
10 0.0143 0.0071 –17.73 11.82 3.55 –2.36
20 0.0071 0.0141 –35.21 23.47 7.04 –4.69
30 0.0214 0.0210 –52.45 34.97 10.49 –6.99
40 0.0286 0.0278 –69.44 46.30 13.89 –9.26
50 0.0357 0.0345 –86.21 57.47 17.24 –11.49