ADA4898-1/ADA4898-2

Data Sheet

Rev. D | Page 14 of 20

THEORY OF OPERATION

The ADA4898 is a voltage feedback op amp that combines unity

gain stability with 0.9 nV/√Hz input noise. It employs a highly

linear input stage that can maintain greater than −90 dBc (at

2 V p-p) distortion out to 600 kHz while in a unity-gain

configuration. This rare combination of unity gain stability, low

input-referred noise, and extremely low distortion is the result

of Analog Devices, Inc., proprietary op amp architecture and

high voltage bipolar processing technology.

The simplified ADA4898 topology, shown in Figure 44, is a

single gain stage with a unity gain output buffer. It has over 100 dB

of open-loop gain and maintains precision specifications, such

as CMRR, PSRR, and offset, to levels that are normally associated

with topologies having two or more gain stages.

BUFFER

R1

Figure 44. Topology

PD (POWER-DOWN) PIN FOR THE ADA4898-1

The PD pin saves power by decreasing the quiescent power

dissipated in the device. It is very useful when power is an issue

and the device does not need to be turned on at all times. The

response of the device is rapid when going from power-down

mode to full power operation mode. Note that PD does not put

the output in a high-Z state, which means that the ADA4898

is not recommended for use as a multiplexer. Leaving the PD

pin floating keeps the amplifier in full power operation mode.

Table 7. Power-Down Voltage Control

PD Pin

±15 V

±10 V

±5 V

Power-Down Mode

≤−14 V

≤−9 V

≤−4 V

0.1 Hz TO 10 Hz NOISE

Figure 45 shows the 0.1 Hz to 10 Hz voltage and current noise

of the ADA4898. The peak-to-peak noise voltage is below 0.5 μV.

Figure 46 shows the circuit used to measure the low frequency

noise. It uses a band-pass filter of approximately 0.1 Hz and 10 Hz

and a high gain stage feeding into an instrumentation amplifier.

0.5

0.4

0.3

0.2

0.1

0

–0.1

–0.2

–0.3

–0.4

–0.5

02

4

6

8

10

12

14

16

18

20

TIME (s)

Figure 45. 0.1 Hz to 10 Hz Noise

DUT

ADA4898-1

+IN

–IN

1µF

10nF

10nF

50Ω

50Ω

+IN

–IN

AD743

1µF

15.8kΩ

13kΩ

13Ω

MOMENTARY

1kΩ

806kΩ

806kΩ

7805

7905

(BATTERY)

(BATTERY)

AD620

–IN

+IN

OUTPUT

REF

R = 5.36kΩ, GAIN APPROX. 10Ω

10nF

1

2

4

3

8

7

5

6

10nF

FLOATING SHIELD

COAX

TEK

TDS 754A SCOPE

IN

FARADAY CAGE

OUT

Figure 46. Low Frequency Noise Circuit