ADA4898-1

Rev. 0 | Page 13 of 16

THEORY OF OPERATION

The ADA4898-1 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

(@ 2 V p-p) distortion out to 500 kHz while in a unity-gain

configuration. This rare combination of low gain stability, low

input referred noise, and extremely low distortion is the result

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

high speed complementary bipolar processing technology.

The simplified ADA4898-1 topology, shown in Figure 40, 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 40. Topology

PD (POWER DOWN) PIN

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 a 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 ADA4898-1 is

not recommended for use as a multiplexer.

CURRENT NOISE MEASUREMENT

To measure the very low (2.4 pA/√Hz) input current noise of

the ADA4898-1, 10 kΩ resistors were used on both inputs of the

amplifier. Figure 41 shows the noise measurement circuit used.

The 10 kΩ resistors are used on both inputs to balance the input

impedance and cancel the common-mode noise. In addition, a

high gain configuration is used to increase the total output noise

and bring it above the noise floor of the instrument.

100Ω

10Ω

10kΩ

10kΩ

Figure 41. Current Noise Measurement Circuit

formula:

[

]

11

kΩ

20

2

)

Hz

nV/

18.4

(11

2

/

1

2

2

×

×

×

−

n

e

I