Design Notes

TI has a number of suitable space-qualified voltage references, but for this application the TL1431-SP was selected for its good noise performance and high radiation tolerance. The lowest voltage that U1 can generate is 2.5 V, so R2 and R3 are used to generate a 1.25-V input to U2 (they also attenuate the noise of U1 by a factor of two). Low values are used for R1 and R2 to limit the noise generated by these components. R1 is chosen so that, even with R2 and R3 connected, there is 1 mA of bias current through U1.

R4 and C1 form the low-pass filter that attenuates noise from the voltage reference. With the values shown, the 3-dB frequency of this filter is 145 Hz. The precise values of R4 and C1 are not critical. However, it pays not to make R4 too large, because the op amp has to correct any errors caused by the output current flowing through R4, and if the voltage drop across R4 is too large the op amp may run out of headroom.

R4 and C1 add a pole to the amplifier response; R5 and C2 add a zero that compensates this pole and enables stable operation. R5 and C2 are chosen so that R5 × C2 = 2 × R4 × C1, which minimizes noise gain peaking in the response.

The op amp used for this application should not itself generate significant noise compared to the voltage reference; however, the filter formed by R4 and C1 attenuates whatever noise the op amp does generate. Note that in the case of the OPA4277-SP, with the component values used, current noise is insignificant compared to voltage noise and can be ignored.