AD537
F-V CONVERTERS
The AD537 can be used as a high linearity VCO in a phase-
locked loop to accomplish frequency-to-voltage conversion. By
operating the loop without a low-pass filter in the feedback path
(first-order system), it can lock to any frequency from zero to an
upper limit determined by the design, responding in three or
four cycles to a step change of input frequency. In practice, the
overall response time is determined by the characteristics of the
averaging filter which follows the PLL.
Figure 7 shows a connection using a low power TTL quad
open-collector nand gate which serves as the phase comparator.
The input signal should be a pulse train or square wave with
characteristics similar to TTL or 5-volt CMOS outputs. Any
duty cycle is acceptable, but the minimum pulse width is 40
µs.
The output voltage is one volt for a 10 kHz input frequency.
The output as shown here is at a fairly high impedance level; for
many situations an additional buffer may be required.
Trimming is similar to V-F application trimming. First set the
V
OS
trimmer to mid-scale. Apply a 10 kHz input frequency and
trim the 2 kΩ potentiometer for 1.00 volts out. Then apply a
10 Hz waveform and trim the V
OS
for 1 mV out. Finally, retrim
the full-scale output at 10 kHz. Other frequency scales can be
obtained by appropriate scaling of timing components.
+5V
normally result in an accuracy of
±
2°C from –55°C to +125°C
(using an AD537S). An NPO ceramic capacitor is recom-
mended to minimize nonlinearity due to capacitance drift.
2kΩ
9.1kΩ
1
LOGIC
GND
10
9
f = 10Hz/K
10kΩ
8 +V
S
CURR-
TO-FREQ
CONV
7
1000pF
6
5
–V
S
(CONNECTED TO CASE)
AD537
DRIVER
+V
2
BUF
3
V
TEMP
V
REF
4
V
T
PRECISION
VOLTAGE
V
R
REFERENCE
Figure 8. Absolute Temperature to Frequency Converter
OFFSET TEMPERATURE SCALES
Many other temperature scales can be set up by offsetting the
temperature output with the voltage reference output. Such a
scheme is shown by the Celsius-to-frequency converter in
Figure 9. Corresponding component values for a Fahrenheit-to-
frequency converter which give 10 Hz/°F are given in parentheses.
AD537
1
2
DRIVER
14
10k
13
12
CURR-
BUF TO-FREQ
CONV
11
10
V
T
V
R
PRECISION
VOLTAGE
REFERENCE
9
8
3900 pF
(1500pF)
f
OUT
10Hz/°C
(10Hz/°F)
+5V
f
IN
(0-10kHz)
AD537
1
2
3
4
9.09k
2k
5
6
7
DRIVER
CURR
BUF -TO-
FREQ
CONV
14
10k
13
12
11
10
9
8
1N4148
20k
V
OS
0.001µF
10k
10k
74LO3
10k
49Ω
(205Ω)
500Ω
6.04k
(10k) 2k
2.74k
(4.02k)
3
4
5
6
7
PRECISION
VOLTAGE
REFERENCE
0.005µF
3.9k
120k
0.33µF
OUTPUT
1V F.S.
Figure 7. 10 kHz F-V Converter
TEMPERATURE-TO-FREQUENCY CONVERSION
Figure 9. Offset Temperature Scale Converters Centigrade
and (Fahrenheit) to Frequency
The linear temperature-proportional output of the AD537 can
be used as shown in these applications to perform various direct
temperature-to-frequency conversion functions; it can also be
used with other external connections in a temperature sensing
or compensation scheme. If the sensor output is used externally,
it should be buffered through an op amp since loading that
point will cause significant error in the sensor output as well as
in the main V-F converter circuitry.
An absolute temperature (Kelvin)-to-frequency converter is very
easily accomplished, as shown in Figure 8. The 1 mV per K out-
put serves as the input to the buffer amplifier, which then scales
the oscillator drive current to a nominal 298
µA
at +25°C
(298K). Use of a 1000 pF capacitor results in a corresponding
frequency of 2.98 kHz. Setting the single 2 kΩ trimmer for the
correct frequency at a well-defined temperature near +25°C will
A simple calibration procedure which will provide
±
2°C accu-
racy requires substitution of a 7.27k resistor for the series com-
bination of the 6.04k with the 2k trimmer; then simply set the
500
Ω
trimmer to give 250 Hz at +25°C.
High accuracy calibration procedure:
1. Measure room temperature in K.
2. Measure temperature output at Pin 6 at that temperature.
3. Calculate offset adjustment as follows:
Offset Voltage ( mV ) =
V
TEMP
(
Pin 6 ) ( mV )
×
273.2
Room temp ( K )
4. Temporarily disconnect 49
Ω
resistor (or 500
Ω
pot) and
trim 2 kΩ pot to give the offset voltage at the indicated node.
Reconnect 49
Ω
resistor.
5. Adjust slope trimmer to give proper frequency at room tem-
perature (+25°C = 250 Hz).
Adjustment for
°F
or any other scale is analogous.
–6–
REV. C
AD [ ANALOG DEVICES ]
