April 10, 1997

Chopping reduces reference’s supply current

Hartmut Henkel, von Hoerner & Sulger GmbH, Schwetzingen, Germany

The 10V precision reference, IC₃, in Figure 1 has temperature drift of approximately 0.6 ppm/°C and good long-term stability. However, because of its zener-based design, this reference needs more than 4 mA of supply current. To reduce this current, the circuit in Figure 1 supplies the reference with chopped power. This scheme lowers the mean supply current according to the duty cycle of the chopping circuit.

To retain a dc reference output, the circuit includes an S/H function. The chopping-control and S/H circuitry add to the supply-current overhead, but this low-power design still minimizes the supply current. The whole circuit, including the reference, draws less than 600 µA, with one op amp of the dual OP295 left unused. The VRE310 reference is stable in this mode, and the 1-kHz chopping frequency maintains on-chip thermal equilibrium.

A polyphase oscillator using the CMOS Schmitt-trigger gates of IC₁ controls the timing. Surprisingly, a single CMOS Schmitt-trigger oscillator needs about 0.5 mA of current, even though the data sheet states a value of less than 5 µA for logic operation. The reason is simple: While sweeping the hysteresis at the gate input, two complementary FETs are conducting. Therefore, at the most power-critical place of the chopping control, this circuit replaces the CD4093 Schmitt-trigger gate with a single CD4007 MOSFET inverter (IC_2C).

R₃ and C₃ determine the chopping frequency. R₁ and C₁ set the delay between power-on and sampling, which IC₃ needs for proper settling, to 25 µsec. R₂ and C₂ set the sampling duration to 50 µsec. R₄ and C₄ ensure start-up of the oscillator and have no other influence, as long as R₄C₄>>R₃C₃.

IC_2A gates the supply of IC₃, and IC_2B samples the reference voltage. Tantalum-capacitor C₅ stores the result, and this capacitor must have very low leakage. A subsequent noise-filtering circuit consists of half of a low-power precision op amp (IC_4A).

The circuit retains the drift and long-term stability of the reference source, but the chopping action somewhat raises the low-frequency voltage noise. The sampling process mixes down non-1/f noise components into the baseband around 0 Hz. However, the peak-to-peak noise voltage, ranging from 0.1 to 10 Hz, measured over 10 sec, is still within an 8-µV band.

This circuit may also work with other references. If you use it with sampled-data, ADC-based systems, you should derive the PWR and sample pulses from the system clock for minimum interference. (DI #2014)

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