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AD537

Integrated Circuit Voltage-to-Frequency Converter

Analog Devices 

Applying the AD537

CIRCUIT OPERATION

Block diagrams of the AD537 are shown above. A versatile

operational amplifier (BUF) serves as the input stage; its pur-

pose is to convert and scale the input voltage signal to a drive

current in the NPN follower. Optimum performance is achieved

when, at the full-scale input voltage, a 1 mA drive current is

delivered to the current-to-frequency converter. The drive cur-

rent to the current-to-frequency converter (an astable

multivibrator) provides both the bias levels and the charging

current to the externally connected timing capacitor. This

“adaptive” bias scheme allows the oscillator to provide low non-

linearity over the entire current input range of 0.1 µA to

2000 µA. The square wave oscillator output goes to the output

driver which provides a floating base drive to the NPN power

transistor. This floating drive allows the logic interface to be ref-

erenced to a different level than –VS. The “SYNC” input (“D”

package only) allows the oscillator to be slaved to an external

master oscillator; this input can also be used to shut off the

oscillator.

The reference generator uses a bandgap circuit (this allows

single-supply operation to 4.5 volts which is not possible with

low T.C. Zeners) to provide the reference and bias levels for the

amplifier and oscillator stages. The reference generator also pro-

vides the precision, low T.C. 1.00 volt output and the VTEMP

output which tracks absolute temperature at 1 mV/K.

V-F CONNECTION FOR POSITIVE INPUT VOLTAGES

The positive voltage input range is from –VS (ground in single

supply operation) to 4 volts below the positive supply. The con-

nection shown in Figure 1 provides a very high (250 MΩ) input

impedance. The input voltage is converted to the proper drive

current at Pin 3 by selecting a scaling resistor. The full-scale

current is 1 mA, so, for example a 10 volt range would require a

nominal 10 kΩ resistor. The trim range required will depend on

capacitor tolerance. Full-scale currents other than 1 mA can be

chosen, but linearity will be reduced; 2 mA is the maximum

allowable drive.

As indicated by the scaling relationship in Figure 1, a 0.01 µF

timing capacitor will give a 10 kHz full-scale frequency, and

0.001 µF will give 100 kHz with a 1 mA drive current. The

maximum frequency is 150 kHz. Polystyrene or NPO ceramic

capacitors are preferred for T.C. and dielectric absorption;

polycarbonate or mica are acceptable; other types will degrade

linearity. The capacitor should be wired very close to the

AD537.

1

GUARD RING 2

3

R2 R1

4

OPTIONAL

INPUT VIN 10kΩ

FILTER

5

10µF

6

7

AD537

DRIVER

FO

=

10

VIN

(R1 + R2)

C

14

fOUT

ROUT

13

+VS

12

BUF

CURR-

TO-FREQ

11

C

CONV

10

VT PRECISION

9

RT

20k

VOLTAGE

VR REFERENCE

8

Figure 1. Standard V-F Connection for Positive Input

Voltages

V-F CONNECTIONS FOR NEGATIVE INPUT VOLTAGE

OR CURRENT

A wide range of negative input voltages can be accommodated

with proper selection of the scaling resistor, as indicated in Fig-

ure 2. This connection, unlike the buffered positive connection,

is not high impedance since the 1 mA F.S. drive current must be

supplied by the signal source. However, very large negative volt-

ages beyond the supply can be handled easily; just modify the

scaling resistors appropriately. Diode CR1 (HP50822811) is

necessary for overload and latchup protection for current or

voltage inputs.

If the input signal is a true current source, R1 and R2 are not

used. Full-scale calibration can be accomplished by connecting a

200 kΩ pot in series with a fixed 27 kΩ from Pin 7 to –VS (see

calibration section, below).

FOUT

=

IIN

10C

0 TO –1mA

IIN

R1

CR1

R2

VIN

0 TO –10V

AD537

FO

=

10

VIN

(R1 + R2)

C

1

14

fOUT

5kΩ (TYP)

2

DRIVER 13

+VS

3

12

4

BUF

CURR-

TO-FREQ

11

C

CONV

5

10

6

VT PRECISION

9

20kΩ

VOLTAGE

7

VR REFERENCE

8

Figure 2. V-F Connections for Negative Input Voltage or

Current

CALIBRATION

There are two independent adjustments: scale and offset. The

first is trimmed by adjustment of the scaling resistor R and the

second by the (optional) potentiometer connected to +VS and

the VOS pins (“D” package only). Precise calibration requires the

use of an accurate voltage standard set to the desired FS value

and a frequency meter; a scope is useful for monitoring output

waveshape. Verification of linearity requires the availability of a

switchable voltage source (or a DAC) having a linearity error

below ± 0.005%, and the use of long measurement intervals to

minimize count uncertainties. Every AD537 is automatically tested

for linearity, and it will not usually be necessary to perform this

verification, which is both tedious and time-consuming.

Although drifts are small it is good practice to allow the operat-

ing environment to attain stable temperature and to ensure that

the supply, source and load conditions are proper. Begin by set-

ting the input voltage to 1/10,000 of full scale. Adjust the offset

pot until the output frequency is 1/10,000 of full scale (for ex-

ample 1 Hz for FS of 10 kHz). This is most easily accomplished

using a frequency meter connected to the output. Then apply

the FS input voltage and adjust the gain pot until the desired FS

frequency is indicated. In applications where the FS input is

small, this adjustment will very slightly affect the offset voltage,

due to the input bias current of the buffer amplifier. A change of

l kΩ in R will affect the input by approximately 100 µV, which is

as much as 0.1% of a 100 mV FS range. Therefore, it may be

necessary to repeat the offset and scale adjustments for the high-

est accuracy. The design of the input amplifier is such that the

input voltage drift after offset nulling is typically below l µV/°C.

REV. C

–3–

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