First Stage High Pass
Filter: Noise of resistor at first stage high pass filter is
critical as this noise directly couples with the reference noise. A small
value resistor (50 ohms) and a bulk capacitor (32 mF) was chosen to design
the first stage high pass filter to minimize the thermal noise from the
resistor. A Vishay bulk metal foil resistor was chosen for first stage
0.1-Hz high-pass filter as the resistor has the best noise figure by
construction. Two 100-Ω bulk metal foil resistors (0.25 W) are kept in
parallel to reduce the impact of current (excess, flicker) noise as this
noise can be reduced by reducing the current and increasing the volume of
resistive material. Four low-leakage electrolyte capacitors are used to
realize 32 mF capacitance with a stable capacitance over an entire frequency
range. These bulk capacitors need a minimum of 12 hours dielectric soaking
time to provide stable measurement. First stage gain resistors are metal
foil resistors as these resistors are good in terms of noise performance.
Eight OPA189s have been connected in parallel through 50-ohm metal film
resistors to reduce the noise by sqrt (8) times.
Later Stages: The
first stage high pass filter is followed by one Sallen-Key 0.1 Hz HPF with
gain 10 and 2 stages of multi-feedback low-pass filter with over all gain 10
and 10 Hz cutoff frequency. The signal is already amplified by 1000 times,
which allows relaxation in component selection for later stages. The
resistance is higher and capacitance is lower in later stages as thermal
noise of the resistors is suppressed by greater than or equal to 100 times.
Please refer to Table 5-1 for the exact values. All the capacitors are film capacitors as the aging
spec is tight and property of filter does not change over time. All the
resistors are metal film in this stage as this type exhibits excellent
thermal noise property. OPA189 has been used in all the stages due to the
ultra-low noise and offset stability.
Power Stage: DC power
supply E3631 is used for this measurement. This is fed to low noise LDO
through an LC filter to get a stable power supply with very good PSRR
measurement. This topology makes sures that there is minimum impact of
supply in the measured noise of the reference.