February 2, 1998

Vital-signs monitor consumes less than 50µA

Leonard Schupak, Discovision Associates, Irvine, CA

A remote data-acquisition circuit for monitoring a patient's vital signs--pulse rate, respiration, and temperature--uses very little power as well as inexpensive sensors and circuits (Figure 1). The data-acquisition portion uses the low-cost CD4000 series of CMOS ICs and consumes less than 50 mA from a 3V battery. The circuit uses 50x oversampled sigma-delta ADCs with a PWM/FM system, which combines three data channels on one RF carrier.

In Figure 1, the pulse sensor comprises a pair of conductive plastic electrodes that monitor two EKG lead points across the patient's chest. These electrodes are similar to those used on a popular and effective pulse monitor for athletic activities. The respiration sensor is an Amp (Harrisburg, PA) DT1-028K, which consists of a piezoelectric-film element mounted on a flexible beam that attaches with a chest strap similar to the same athletic monitor. The strap uses the beam as one of its suspension loops, the electronics assembly attaches to the other end, and the complete assembly straps to the patient's chest. The temperature sensor comprises a 50-kilohms thermistor and linearizing resistor mounted in the EKG electrode, which makes a good thermal connection to the patient's body.

Using the unique properties of the CD4069 as an analog component, the circuit implements two second-order sigma-delta ADCs that sample the ac components of the pulse (EKG) and respiration at a nominal 2-kHz clock rate. The circuit develops this clock using the thermistor as a tuning element in a temperature-to-frequency converter. Temperature is a very low-frequency (essentially dc) data-channel component, and pulse and respiration are higher frequency ac. Thus, you can easily combine all three channels in one serial data channel for transmission over the RF link.

The analog circuit also includes an open-lead detector that rings an alarm if the EKG lead loses contact. IC_1C and IC_1F form a Schmitt trigger that disables the RF-carrier output when the sense of skin conductivity is lost.

IC_2A and IC_2D form a straightforward clock oscillator. Although easy to design, this oscillator imposes some interesting calibration problems in operation. The thermistor, R₁, and tuning capacitor, C₁, determine the frequency. For a thermistor of 11 kilohms at 98.6°F, C₁ should be 0.022 µF for a clock frequency of 2 kHz. Connecting these components to IC_2A and IC_2D, as the figure shows, provides a convenient pc-board layout with good isolation from these low-level input signals. You can use an additional resistor set to optimize the frequency range, depending on the thermistor selection. The operator usually performs a calibration cycle when you apply the unit to the patient, setting the "current reading" to a known temperature obtained directly from the patient.

The operation of the ADC is straightforward but has some unusual aspects. (Note that because of the way the CD4069 inverter works--used here exclusively for analog-amplifier functions--the figure uses a one-input NAND gate as the logic symbol to differentiate from the triangle symbol.) An ADC channel comprises three CD4069 inverters in series, for which two operate as analog integrating amplifiers biased in the linear region. In this configuration, each CD4069 section yields about 30 dB of gain and a bandwidth in excess of 2 kHz. The third inverter and flip-flop input act as the typical comparator part of the ADC.

The first stage is a relatively uncomplicated integrator with an indeterminate time constant of approximately 1 to 10 µsec. The second stage has a pole/zero response that flattens to ­20 dB at approximately 5 kHz. Feedback from the sampling register (flip-flop) provides a stable, wideband inner loop as well as the main outer loop. The 100-megohms resistors in the feedback loops are readily available from several vendors in surface-mount configuration and are reliable when you use good assembly practices.

The components in the figure comprise only the front-end input portion of the design; the noise filter and subsequent decoder/application circuits reside at the receiver. Before transmission, the circuit combines the three channels of data into one modulating signal using a PCM format similar to modified FM (MFM) for which the temperature-controlled clock oscillator determines the pulse-rate clock. In other words, the transmitter uses a complex set of modulating codes that include a PCM/FM component superimposed on an amplitude-shift-keyed (ASK) carrier. The temperature-sensitive oscillator frequency varies about 20% total over the expected range of body temperatures, and this variable rate is adequate to transmit the digital PCM data.

A CD4008 full adder encodes the digital data from each ADC flip-flop in MFM form. A simple 3-bit digital synthesizer accomplishes the encoding; the contents of output registers IC_3C and IC_3D determine the output phase. The ADC's data contents determine the incremental phase change at each clock (Table 1). The circuit then uses the most significant bit of the output register as the ASK signal. The 4-bit register then stores the sum of the data, or the phase increment with the current contents, representing the digital phase of the modulation carrier. The maximum resulting fundamental frequency of this carrier is then one-half the clock frequency.  

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