Datasheet AD537 (Analog Devices) - 6

AD537 F-V CONVERTERS
normally result in an accuracy of ± 2°C from –55°C to +125°C The AD537 can be used as a high linearity VCO in a phase- (using an AD537S). An NPO ceramic capacitor is recom- locked loop to accomplish frequency-to-voltage conversion. By mended to minimize nonlinearity due to capacitance drift. 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
LOGIC GND
upper limit determined by the design, responding in three or
2k
Ω
10 9.1k
Ω four cycles to a step change of input frequency. In practice, the
1 9 f = 10Hz/K AD537
overall response time is determined by the characteristics of the
10k
Ω
DRIVER
averaging filter which follows the PLL.
+V 2 8 +VS
Figure 7 shows a connection using a low power TTL quad
CURR- TO-FREQ
open-collector nand gate which serves as the phase comparator.
BUF CONV
The input signal should be a pulse train or square wave with
3 7 VT PRECISION
characteristics similar to TTL or 5-volt CMOS outputs. Any
VTEMP VOLTAGE VR
duty cycle is acceptable, but the minimum pulse width is 40 µs.
REFERENCE 1000pF VREF 4 6
The output voltage is one volt for a 10 kHz input frequency.
5
The output as shown here is at a fairly high impedance level; for
–VS
many situations an additional buffer may be required.
(CONNECTED TO CASE)
Trimming is similar to V-F application trimming. First set the V Figure 8. Absolute Temperature to Frequency Converter 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
OFFSET TEMPERATURE SCALES
OS for 1 mV out. Finally, retrim the full-scale output at 10 kHz. Other frequency scales can be Many other temperature scales can be set up by offsetting the obtained by appropriate scaling of timing components. temperature output with the voltage reference output. Such a scheme is shown by the Celsius-to-frequency converter in
+5V
Figure 9. Corresponding component values for a Fahrenheit-to- frequency converter which give 10 Hz/°F are given in parentheses.
fIN (0-10kHz) AD537 74LO3 AD537 f 1 14 OUT 10k 10Hz/
°
C 10k 1 14 2 13 10k (10Hz/
°
F) DRIVER 2 13 0.001µF 10k DRIVER +5V 3 12 CURR 10k 3 12 -TO- 49
Ω
3900 pF 4 BUF 11 FREQ (205
Ω
) CURR- BUF (1500pF) CONV 4 TO-FREQ 11 5 10 9.09k CONV 500
Ω
10 2k 20k 5 6 9 PRECISION VOS VOLTAGE 6.04k 6 V 9 T PRECISION 7 REFERENCE 8 (10k) 2k VOLTAGE 7 VR REFERENCE 8 1N4148 2.74k 0.005µF (4.02k) 3.9k 120k OUTPUT 1V F.S. 0.33µF
Figure 9. Offset Temperature Scale Converters Centigrade and (Fahrenheit) to Frequency Figure 7. 10 kHz F-V Converter A simple calibration procedure which will provide ± 2°C accu-
TEMPERATURE-TO-FREQUENCY CONVERSION
racy requires substitution of a 7.27k resistor for the series com- The linear temperature-proportional output of the AD537 can bination of the 6.04k with the 2k trimmer; then simply set the be used as shown in these applications to perform various direct 500 Ω trimmer to give 250 Hz at +25°C. temperature-to-frequency conversion functions; it can also be High accuracy calibration procedure: used with other external connections in a temperature sensing 1. Measure room temperature in K. or compensation scheme. If the sensor output is used externally, 2. Measure temperature output at Pin 6 at that temperature. it should be buffered through an op amp since loading that 3. Calculate offset adjustment as follows: point will cause significant error in the sensor output as well as V in the main V-F converter circuitry. TEMP ( Pin 6 ) ( mV ) Offset Voltage ( mV ) = × 273.2 Room temp ( K ) An absolute temperature (Kelvin)-to-frequency converter is very 4. Temporarily disconnect 49 Ω resistor (or 500 Ω pot) and easily accomplished, as shown in Figure 8. The 1 mV per K out- trim 2 kΩ pot to give the offset voltage at the indicated node. put serves as the input to the buffer amplifier, which then scales Reconnect 49 Ω resistor. the oscillator drive current to a nominal 298 µA at +25°C 5. Adjust slope trimmer to give proper frequency at room tem- (298K). Use of a 1000 pF capacitor results in a corresponding perature (+25°C = 250 Hz). frequency of 2.98 kHz. Setting the single 2 kΩ trimmer for the Adjustment for °F or any other scale is analogous. correct frequency at a well-defined temperature near +25°C will –6– REV. C