AD823

REV. 0

–13–

Since the input stage uses n-channel JFETs, input current dur-

ing normal operation is negative; the current flows out from the

input terminals. If the input voltage is driven more positive than

device junctions become forward biased. This is illustrated in

Figure 6.

A current limiting resistor should be used in series with the in-

put of the AD823 if there is a possibility of the input voltage ex-

ceeding the positive supply by more than 300 mV, or if an input

voltage will be applied to the AD823 when

±V

fier will be damaged if left in that condition for more than 10

seconds. A 1 k

Ω resistor allows the amplifier to withstand up to

10 volts of continuous overvoltage, and increases the input volt-

age noise by a negligible amount.

The amplifier can safely withstand input voltages 20 volts below

the minus supply voltage as long as the total voltage from the

positive supply to the input terminal is less than 36 volts. In

addition, the input stage typically maintains picoamp level input

currents across that input voltage range.

The AD823 is designed for 16 nV/

√Hz wideband input voltage

noise and maintains low noise performance to low frequencies

(refer to Figure 15). This noise performance, along with the

AD823’s low input current and current noise means that the

AD823 contributes negligible noise for applications with source

resistances greater than 10 k

Ω and signal bandwidths greater

than 1 kHz.

OUTPUT CHARACTERISTICS

The AD823’s unique bipolar rail-to-rail output stage swings

within 25 mV of the supplies with no external resistive load. The

AD823’s approximate output saturation resistance is 25

Ω

sourcing and sinking. This can be used to estimate output satu-

ration voltage when driving heavier current loads. For instance,

when driving 5 mA, the saturation voltage to the rails will be ap-

proximately 125 mV.

If the AD823’s output is driven hard against the output satura-

tion voltage, it will recover within 250 ns of the input returning

to the amplifier’s linear operating region.

A/D Driver

The rail-to-rail output of the AD823 makes it useful as an A/D

driver in a single supply system. Because it is a dual op amp, it

can be used to drive both the analog input of the A/D along with

its reference input. The high impedance FET input of the

AD823 is well suited for minimally loading of high output im-

pedance devices.

Figure 38 shows a schematic of an AD823 being used to drive

both the input and reference input of an AD1672, a 12-bit

3 MSPS single supply A/D converter. One amplifier is config-

ured as a unity gain follower to drive the analog input of the

AD1672 which is configured to accept an input voltage that

ranges from 0 to 2.5 V.

13

14

12

11

10

9

8

7

6

5

4

3

2

1

19

18

+5VA

10µF

0.1µF

2

3

5

6

4

7

1

8

49.9

Ω

10µF

0.1

µF

0.1

µF

10

µF

0.1µF

+5VA

+5VD

+5VD

20

21

22

23

24

25

26

27

16

CLOCK

1k

1k

(1.25V)

BIT1 (MSB)

BIT2

BIT3

BIT4

BIT5

BIT6

BIT7

BIT8

BIT9

BIT10

BIT11

BIT12 (LSB)

15

OTR

REFOUT

AIN1

AIN2

REFIN

IN COM

NCOMP2

NCOMP1

ACOM

COM

REF

D

COM

AD823

28

19

AD1672

Figure 38. AD823 Driving Input and Reference of the

AD1672, a 12-Bit 3 MSPS A/D Converter

The other amplifier is configured as a gain of two to drive the

reference input from a 1.25 V reference. Although the AD1672

has its own internal reference, there are systems that require

greater accuracy than the internal reference provides. On the

other hand, if the AD1672 internal reference is used, the second

AD823 amplifier can be used to buffer the reference voltage for

driving other circuitry while minimally loading the reference

source.

The circuit was tested with a 500 kHz sine wave input that was

heavily low pass filtered (60 dB) to minimize the harmonic con-

tent at the input to the AD823. The digital output of the

AD1672 was analyzed by performing an FFT.

During the testing, it was observed that at 500 kHz, the output

of the AD823 cannot go below about 350 mV (operating with

negative supply at ground) without seriously degrading the sec-

ond harmonic distortion. Another test was performed with a

200

Ω pull-down resistor to ground that allowed the output to

go as low as 200 mV without seriously affecting the second har-

monic distortion. There was, however, a slight increase in the

third harmonic term with the resistor added, but it was still less

than the second harmonic.