AD537

REV. C

–6–

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

trim the 2 k

Ω potentiometer for 1.00 volts out. Then apply a

the full-scale output at 10 kHz. Other frequency scales can be

obtained by appropriate scaling of timing components.

1

2

14

13

5

6

7

10

9

8

3

4

12

11

AD537

DRIVER

PRECISION

VOLTAGE

REFERENCE

CURR

-TO-

FREQ

CONV

BUF

20k

(0-10kHz)

2k

10k

0.001µF

1N4148

0.005µF

3.9k

120k

OUTPUT

1V F.S.

0.33µF

10k

10k

74LO3

+5V

10k

9.09k

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 k

Ω 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.

DRIVER

CURR-

TO-FREQ

CONV

PRECISION

VOLTAGE

REFERENCE

AD537

+V

LOGIC

GND

(CONNECTED TO CASE)

10

9

8

7

6

5

4

3

2

1

9.1k

Ω

10k

Ω

f = 10Hz/K

1000pF

2k

Ω

BUF

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

14

13

5

6

7

10

9

8

3

4

12

11

AD537

10k

10Hz/

°C

(10Hz/

°F)

+5V

2.74k

(4.02k)

500

Ω

3900 pF

(1500pF)

2k

6.04k

(10k)

49

Ω

(205

Ω)

BUF

DRIVER

CURR-

TO-FREQ

CONV

PRECISION

VOLTAGE

REFERENCE

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 ) =

Room temp ( K )

× 273.2

4. Temporarily disconnect 49

Ω resistor (or 500 Ω pot) and

trim 2 k

Ω 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.