AD652
Figure 26. Demultiplexer Frequency-to-Voltage Conversion
Figure 27. Isolated Synchronous VFC
Analog Signal Reconstruction
ISOLATED FRONT END
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.
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
–14–
REV. B
ADI [ ADI ]
