Design Goals

Design Description

This circuit design uses a low-pass filter and an op amp to reduce the wideband noise generated by a shunt voltage reference. The circuit was originally developed to supply a CCD sensor in a satellite: the shunt reference alone does not meet the performance requirements of the application and needs the additional circuitry to meet the project requirements.

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.

Design Steps

• Select resistors R2 and R3 to attenuate the 2.5-V output voltage of the TL1431-SP to 1.25 V. In this case, a value of R2 = R3 = 1 kΩ provides the necessary attenuation and ensures that the thermal noise generated by these resistors is insignificant compared to other noise sources and can be ignored.
• Use the following equation to calculate the current flowing through resistors R2 and R3:
  
  
  

• Use the following equation to calculate the maximum value of R1:

  
  
  

  A value of 3.9 kΩ is therefore suitable for R1.

• Choose values for R4 and C1 that will create a pole at the desired cutoff frequency. In this application, a cutoff frequency of about 150 Hz was selected: this provides sufficient noise attenuation in the frequency range of interest (1 kHz to 10 MHz). The op amp compensates for the voltage drop across R4, so do not make it too big. As a general rule, it is recommended to make sure that the voltage drop across R4 is less than 1 V at maximum output current.

  Use the following equation to calculate the RC time constant needed for the desired cutoff frequency:

  
  
  

  Using values of R4 = 110 Ω and C1 = 10 μF results in a cutoff frequency of 145 Hz.

• To minimize noise gain peaking close to the cutoff frequency, choose R5 and C2 so that: