AD652

REV. B

–14–

Figure 26. Demultiplexer Frequency-to-Voltage Conversion

Figure 27. Isolated Synchronous VFC

Analog Signal Reconstruction

If it is desired to reconstruct the analog voltages from the multi-

plex signal, then three more AD652 SVFC devices are used as

frequency-to-voltage converters, as shown in Figure 26. The

comparator inputs of all the devices are strapped together, and

the “+” inputs are held at a 1.2 volt TTL threshold, while the

“–” inputs are driven by the multiplex input. The three clock

inputs are driven by the

φ outputs of the clock chip. Remember

that data at the comparator input of the SVFC is loaded on the

falling edge of the clock signal and shifted out on the next rising

edge. Note that the frequency signals for each data channel are

available at the frequency output pin of each FVC.

ISOLATED FRONT END

In some applications it may be necessary to have complete

galvanic isolation between the analog signals being measured and

the digital portions of the circuit. The circuit shown in Figure

27 runs off a single 5 volt power supply and provides a self-

contained, completely isolated analog measurement system. The

power for the AD652 SVFC is provided by a chopper and a

transformer, and is regulated to

±15 volts.

Both the chopper frequency and the AD652 clock frequency are

125 kHz, with the clock signal being relayed to the SVFC through

the transformer. The frequency output signal is relayed through