AD537 Datasheet, PDF (6/8 Pages) Analog Devices – Integrated Circuit Voltage-to-Frequency Converter

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
9.09k
2k
f_IN
(0-10kHz)
AD537
1
14
10k
10k
2
DRIVER ¹³
0.001µF ^10k
3
12
CURR
4
BUF
-TO-
FREQ
11
CONV
5
10
6
PRECISION ⁹
VOLTAGE
20k
V_OS
7
REFERENCE 8
1N4148
0.005µF _3.9k
120k
OUTPUT
1V F.S.
0.33µF
74LO3
10k
Figure 7. 10 kHz F-V Converter
TEMPERATURE-TO-FREQUENCY CONVERSION
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
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
+V
9.1kΩ
2
LOGIC
GND
10
1
9
AD537
DRIVER
BUF
CURR-
TO-FREQ
CONV
3
V_TEMP
V_{T PRECISION}
VOLTAGE
V_{R REFERENCE}
V_REF 4
6
5
–V_S
(CONNECTED TO CASE)
f = 10Hz/K
10kΩ
8 +VS
7
1000pF
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.
1
2
49Ω
3
(205Ω)
4
500Ω
5
AD537
14
DRIVER ¹³
12
BUF
CURR-
TO-FREQ
11
CONV
10
f_OUT
10Hz/°C
10k
(10Hz/°F)
+5V
3900 pF
(1500pF)
6.04k
6
V_T PRECISION
9
(10k) 2k
VOLTAGE
7
V_{R REFERENCE}
8
2.74k
(4.02k)
Figure 9. Offset Temperature Scale Converters Centigrade
and (Fahrenheit) to Frequency
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 ) = ^{VTEMP ( 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