4
Channel Frequency to Voltage Converter
Description
The 5636 is a programmable
four-channel dual-input Frequencyto-
Voltage converter that provides
signal conditioning for frequency
output transducers. This includes
magnetic pickups, optical encoders,
and other rotation or position-sensing
devices. The input pulses
are converted to a filtered DC
output voltage between 0 and ±10V,
based on the input frequency, the
user-selected Hz/V conversion factor,
and the direction of rotation.
Design Features
The 5636 provides input signal
isolation to prevent ground-loop
problems, programmable gain to
accommodate input signal levels from
25 mV to 250 Volts, a secondary fully
conditioned input for direction-sense
which configures the output voltage
polarity to indicate forward or reverse
rotation, programmable low-pass
output filtering, and programmable
excitation voltage. To assist with
setup and troubleshooting, the
conditioned input pulses can be
monitored at the DC output
connector. Front-panel LEDs indicate
signal presence and activity level, and
warn of module operational problems.
To verify channel operation and
accuracy, an on-board programmable
frequency generator can be
switched into the input with a high accuracy
24 bit A/D converter used
to calibrate the analog outputs. Low
temperature-coefficient components
are used to maintain system accuracy
over a wide temperature range, and
all circuitry is housed in a shielded
enclosure for improved reliability and noise reduction.
Dual Input Stages
Each sensor channel contains
conditioning for two isolated signal
sources allowing for direction sensing
in addition to base frequency-to-voltage conversion. Both signal inputs
are zener-clipped and fed through
isolation transformers to prevent
ground-loop problems and provide up
to 250V of common-mode rejection.
For input frequencies below 100 Hz
(the isolation transformer minimum),
a bypass relay is provided for direct
coupling to each wave-shaper
amplifier.
Wave Shaper
This stage converts input signals of various amplitudes and
waveshapes into a conditioned square wave. The signal is first
amplified to a suitable level using a programmable gain amplifier
and then converted to a square wave using a comparator with
programmable voltage hysteresis. The shaped signal is then
fed to the DSP for processing, and to an output multiplexer,
allowing direct output of the shaped input signal.
DSP Frequency Conversion
The Digital Signal Processor is the heart of the 5636 Frequency
to Voltage converter. Using an internal reference clock that
is accurate to 0.001%, the input signal frequency is accurately
measured and converted to an equivalent 0 to ±10V digital value,
based on the user-selected conversion range and the calibration
constants stored in memory. The digital values generated by
the DSP are then averaged and filtered according to user-selected
parameters, including output bandwidth and slew rate. This
allows for a wide conversion range without sacrificing accuracy
or depending on the tolerances of matched analog components.
Output Circuit
This circuit consists of a 16-bit Digital-to-Analog Converter
followed by a four-pole smoothing filter which removes any
digital steps in the output signal generated by the DSP filter
and skew rate controls. In addition, an output multiplexer
allows the user to monitor either the DC output voltage or
shaped input signals.
Voltage Excitation with Remote Sense Capability
The 5636 provides four independent programmable excitation voltage sources for use in powering input sensors. Each source is controlled by a 16 bit DAC with an output range from 0 to 10V and 100mA output buffering. Remote sense can also be selected to provide feedback accounting for IR losses over conductor lines between amplifier and sensor.
System Calibration
High accuracy is obtained during the conversion process
by implementing a unique end-to-end calibration scheme within
the 5636 converter. The DSP chip generates a calibration
signal that is accurate to 0.001%. This signal is
switched into the input, processed by the normal
input wave-shaping circuit, counted by the DSP processor,
and converted to an analog output voltage by the
D/A converter and smoothing filter. An accurate 24-bit
A/D converter is then used to measure the actual
DC output voltage, allowing correction factors to
be calculated and stored in the DSP memory for enhanced
conversion accuracy. |
5636
Specifications
Schematic
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