A voltage-to-frequency converter (VFC) can easily measure current flow and convert it to a digital signal understood by the timer/counter input of a microcomputer (µC) or microprocessor. It's a straightforward task to add a single opto-isolator in the signal chain when electrical isolation is needed between the current flow and the µC. This circuit demonstrates the use of an AD7740 synchronous VFC for low-side current measurements.
In Figure 1, the current flow both into and out of a rechargeable battery is being monitored. Since the current through the sense resistor (RS) can be bidirectional, a bipolar voltage signal with respect to GND will develop across RS. Although designed for positive-only analog inputs, the AD7740 can accept and convert negative voltage levels down to 150 mV.
Nonlinearities increase below this level due to the increased leakage of the analog input protection diode. To work with negative input voltages, the high-impedance input buffer must be turned off by tying the BUF pin low. The input impedance with the buffer off is still about 600 k, rendering the loading effect on the sense resistor negligible. The transfer function for the AD7740 is:
fOUT = 0.1fCLKIN + 0.8(VIN/VREF)FCLKIN
where fCLKIN is the input clock frequency, VIN is the input voltage, and VREF is the reference voltage.
X1 is a low cost, 1-MHz ceramic resonator, such as the Murata CSBF1000J. It's used with the internal oscillator circuit to provide the clock source. Since an absolute reading of the current flow is required, the 2.5-V internal voltage reference is not used in this application.
Instead, the REFIN/OUT pin is tied to an AD589 with a 1.235-V output. If the 3.3-V supply to the AD7740 is tightly regulated, then the REFIN/OUT pin can be tied to VDD directly. With a 10-m sense resistor, the VFC input voltage varies ±100 mV for a ±10-A current flow.
With a 0-V input, output offset measures approximately 6 kHz from the ideal 100 kHz. This offset is constant over the full signal range and doesn't contribute to nonlinearity. Under these conditions, the output frequency varies from 41 kHz with a -100-mV input to 170.9 kHz with a 100-mV input (Fig. 2).
Thus, the output frequency varies by approximately ±64.5 kHz around 106 kHz for an input change of ±100 mV from 0 V. This circuit's accuracy is better than 0.2% of full scale. The value of the sense resistor is scaled according to the maximum bidirectional current to be measured, and this value can be increased to 100 m to monitor input currents of ±1 A.
