AD824

–12–

REV. A

APPLICATIONS

Single Supply Voltage-to-Frequency Converter

The circuit shown in Figure 31 uses the AD824 to drive a low

positive going output pulse is integrated by R1-C1 and used as

one input to the AD824, which is connected as a differential

integrator. The other input (nonloading) is the unknown volt-

ing the overall feedback loop.

2

6

5

3

4

+10V

0.1

µF

C5

R

10k

R1

499k, 1%

R2

499k, 1%

0V TO 2.5V

FULL SCALE

C1

0.01

µF, 2%

C2

0.01

µF, 2%

U4

REF02

U1

43

U3B

21

U3A

C6

390pF

5%

C3

0.1

µF

THR

TR

DIS

RV+

OUT

CV

GND

48

6

2

7

1

3

5

(NPO)

C4

0.01

µF

R3

*

116k

U2

CMOS 555

OUT2

OUT1

NOTES:

f

/(VREF*t

°C TC

°C TC

t

= 2.0V

= 25kHz f

V

CMOS

74HCO4

1/4

AD824B

V

IN

V

IN

V

IN

Figure 31. Single Supply Voltage-to-Frequency Converter

Typical AD824 bias currents of 2 pA allow megaohm-range

source impedances with negligible dc errors. Linearity errors on

the order of 0.01% full scale can be achieved with this circuit.

This performance is obtained with a 5 volt single supply, which

delivers less than 3 mA to the entire circuit.

Single Supply Programmable Gain Instrumentation Amplifier

The AD824 can be configured as a single supply instrumenta-

tion amplifier that is able to operate from single supplies down

to 3 V or dual supplies up to

±15 V. AD824 FET inputs’ 2 pA

bias currents minimize offset errors caused by high unbalanced

source impedances.

An array of precision thin-film resistors sets the in amp gain to

be either 10 or 100. These resistors are laser-trimmed to ratio

match to 0.01% and have a maximum differential TC of

5 ppm/

°C.

Table I. AD824 In Amp Performance

Parameters

5 V

CMRR

74 dB

80 dB

Common-Mode

Voltage Range

–0.2 V to +2 V

–5.2 V to +4 V

3 dB BW, G = 10

180 kHz

180 kHz

G = 100

18 kHz

18 kHz

2

µs

5

µs

Noise @ f = 1 kHz, G = 10

270 nV/

√Hz

270 nV/

√Hz

G = 100

2.2

µV/√Hz

2.2

µV/√Hz

10

0%

100

90

1V

5µs

Figure 32a. Pulse Response of In Amp to a 500 mV p-p

(G =10)

V

) +V

REF

R6

R4 + R5

(G =100)

V

) +V

REF

FOR R1 = R6, R2 = R5 AND R3 = R4

R5 + R6

R4

R1

R2

R3

R4

R5

R6

90k

9k

1k

1k

9k

90k

V

OUT

V

IN1

0.1

µF

8

1/4

AD824

1/4

AD824

V

IN2

R

P

1k

Ω

1

2

3

6

5

11

7

OHMTEK

PART # 1043

V

REF

G =10

G =100

R

P

1k

Ω

G =100

G =10

+V

S

Figure 32b. A Single Supply Programmable

Instrumentation Amplifier