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EDN Access— 01.19.95 Converter ignores power-supply variation

-January 05, 1995

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Design Ideas:January 5, 1995

Converter ignores power-supply variations


Samuel Kerem,
Infrared Fiber Systems Inc, Silver Spring, MD

The simple period-to-frequency converter in Fig 1 uses a dual-slope ADC (IC2) to drive a 3.5-digit display. One medical application uses the circuit to convert the time interval between two heartbeats to a rate of number per minute.

The display reading representing the measurement value is

N = K x VIN/VREF,

where N is the measurement reading, K is a constant, and VIN and VREF are the ADC's input and reference signals, respectively. In Fig 1,


Each incoming pulse resets the integrator comprising IC1, C1, and R4 by closing switch Q1. The circuit connects the integrator's output to C2 by closing switch Q2 prior to reset. C2 holds the reference voltage

VREF = VOUT =I x T/C1

Assuming R4 is much greater than R3,


because


Substituting back to Eq 1 results in


or

N = K1/T,

where the period T equals t and N equals the frequency. Note: VCC drops out in this final expression because the converter isn't sensitive to power-supply variations. In addition, the device does not require a stable reference source.

Pulse duration is a major source of nonlinearity. Therefore, you should keep the pulses as short as possible but still long enough to discharge C1 and recharge C2. The low bias current of the ADC's reference input and low leakage current through Q2 allow negligible discharge in a few seconds. Optional R5 helps to filter out rapid frequency changes in consecutive pulses. The time between two consecutive heartbeats can differ from the average by ±10% during a 1-minute interval.

For simplicity, Fig 1 doesn't include all the standard and additional components for the real circuit's operation. You need only consider the stability of R2, R3, R4, and C1. The display data depends linearly on the value of R4, which you can use as a single element for calibration. You should choose the values for R2, R3, R4, and C1 to provide an integrator-output signal close to saturation for the longest possible interval between incoming pulses. (DI #1641)


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