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

6.1 Calculating Reference Source Impedance Error in Difference Amplifier

For the single-stage difference amplifier, all that is needed is the values of the internal resistors. Sometimes these values are either partially or entirely listed in data sheets. For example, Figure 6-1 is listed in the INAx181 data sheet.

Figure 6-1 INAx181 Input Resistance

"RINT" in this table represents the input resistors, and corresponds to Ri in Figure 3-1 and Figure 5-2. The feedback resistor is not listed. However, inferences can be based on the information available. For example, the feedback resistor, Rf, in INAx181A2 can be found:

Equation 20. R f = R i × 50 = 500   k Ω

Alternatively, the combined resistance of Ri + Rf can be measured. In case no resistance information is given in the data sheet, this method can be used to find the values of Ri and Rf. As an example, the total resistance turns out to be 494 kΩ for a sample INA181A2. Since the ratio of the two resistors equals to the gain, the resistance values can be calculated:

Equation 21. R i = 494   k Ω 51 = 9.7   k Ω           a n d       R f = 50 × 494   k Ω 51 = 484.3   k Ω

Table 6-1 shows calculated output error terms (columns titled "Err_Vref", "Err_Vcm", and "Err_Vdiff") due to finite source impedance Rx, using these measured resistance values. The external condition is: Vref_x = 2.5 V; Vcm = 25 V; Vdiff = 50 mV. The last column, "Err_Total" is the sum of all three error terms.

Table 6-1 Reference Source Impedance Error for INA181A2
Rx (kΩ) C = Rx / (Ri + Rf) m = C / (1 + C) Err_Vref (mV) Err_Vcm (mV) Err_Vdiff (mV) Err_Total (mV)
1 0.0020 0.0020 –5.05 50.54 0.10 45.59
5 0.0101 0.0100 –25.07 250.66 0.50 226.10
10 0.0203 0.0199 –49.63 496.35 0.99 447.70
20 0.0405 0.0389 –97.34 973.37 1.95 877.98
30 0.0608 0.0573 –143.22 1432.17 2.86 1291.82
40 0.0810 0.0750 –187.38 1873.78 3.75 1690.15
50 0.1013 0.0920 –229.91 2299.15 4.60 2073.83