AD650

Data Sheet

Rev. E | Page 16 of 20

APPLICATIONS

DIFFERENTIAL VOLTAGE-TO-FREQUENCY

CONVERSION

The circuit in Figure 20 accepts a true floating differential input

+15 V to −5 V with respect to analog ground. The signal input,

inputs are low impedance; the source that drives the common-

mode input must supply the 0.5 mA drawn by the bipolar offset

current source, and the source that drives the signal input must

supply the integration current.

If less common-mode voltage range is required, then a lower

voltage Zener can be used. For example, if a 5 V Zener is used,

not used at all, the common-mode range is ±5 V with respect to

analog ground. If no Zener is used, the 10 kΩ pulldown resistor

is not needed and the integrator output (Pin 1) is connected

directly to the comparator input (Pin 9).

AUTOZERO CIRCUIT

In order to exploit the full dynamic range of the AD650 VFC,

very small input voltages need to be converted. For example, a

six decade dynamic range based on a full scale of 10 V requires

accurate measurement of signals down to 10 μV. In these

situations, a well-controlled input offset voltage is imperative. A

constant offset voltage does not affect dynamic range but simply

shifts all of the frequency readings by a few hertz. However, if

the offset should change, it is not possible to distinguish

between a small change in a small input voltage and a drift of

the offset voltage. Therefore, the usable dynamic range is less.

The circuit shown in Figure 21 provides automatic adjustment

of the op amp offset voltage. The circuit uses an AD582 sample-

and-hold amplifier to control the offset, and the input voltage to

the VFC is switched between ground and the signal to be

measured via an AD7512DI analog switch. The offset of the

AD650 is adjusted by injecting a current into—or drawing a

current out of—Pin 13. Note that only one of the offset null pins

is used. During the VFC norm mode, the SHA is in the hold

mode and the hold capacitor is very large, 0.1 μF, which holds

the AD650 offset constant for a long period of time.

When the circuit is in the autozero mode, the SHA is in sample

mode and behaves like an op amp. The circuit is a variation of

the classical two amplifier servo loop, where the output of the

device under test (DUT)—here the DUT is the AD650 op

amp—is forced to ground by the feedback action of the control

amplifier—the SHA. Because the input of the VFC circuit is

connected to ground during the autozero mode, the input

current that can flow is determined by the offset voltage of the

AD650 op amp. Because the output of the integrator stage is

forced to ground, it is known that the voltage is not changing (it

is equal to ground potential). Therefore, if the output of the

integrator is constant, its input current must be zero, so the

offset voltage has been forced to be zero. Note that the output of

the DUT could have been forced to any convenient voltage

other than ground. All that is required is that the output voltage

be known to be constant. Note also that the effect of the bias

current at the inverting input of the AD650 op amp is also

mulled in this circuit. The 1000 pF capacitor shunting the

200 kΩ resistor is compensation for the two amplifier servo

loop. Two integrators in a loop require a single zero for

compensation. The 3.6 kΩ resistor from Pin 1 of the AD650 to

the negative supply is not part of the autozero circuit, but rather,

it is required for VFC operation at 1 MHz.

OP

AMP

COMP

IN

FREQ

OUT

OUT

ONE

SHOT

8

9

10

11

13

12

7

6

5

4

3

2

1

14

INPUT

OFFSET

TRIM

10V ZENER 1N5240

NOTES

+15V TO –5V WITH RESPECT TO ANALOG GROUND.

–0.6V

AD650

INPUT

1mA

S1

20kΩ

250kΩ

0.1µF

1µF

+15V

GND

GND

FREQUENCY

OUTPUT

0kHz TO 100kHz

–15V

+5V

–

+

–

+

0.1µF

–

+

1kΩ

10kΩ

330pF

1000pF

1.24kΩ

40kΩ

10kΩ

Figure 20. Differential Input