ADA4099-1BUJZ-R5 datasheet(22/34 Pages) AD

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Data Sheet

ADA4099-1/ADA4099-2

THEORY OF OPERATION

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Rev. A | 22 of 34

This RIN resistance appears across the summing nodes in Over-

The-Top operation due to the configuration of the common base

input stage.

The RIN value is derived from the specified IB that flows to the op

amp inputs, as expressed in the following equation:

RIN = 2kT/(qIB)

where:

k is Boltzmann’s constant.

T is the operating temperature.

q is the charge of an electron.

IB is the operating input bias current in Over-The-Top operation.

The inputs are biased proportional to absolute temperature. There-

fore, RIN is relatively constant with temperature. This resistance

appears across the summing nodes of the amplifiers, which is

forced to 0 V differentially by the feedback action of the amplifi-

ers and can seem relatively harmless. However, depending on

the configuration, this input resistance can boost the noise gain,

lower overall amplifier loop gain and closed-loop bandwidth, and

raise output noise. The singular benefit of this configuration is an

increase in closed-loop amplifier stability.

In normal mode (−VS < VCM < +VS −1.5 V), RIN is typically large

compared to the value of the gain setting resistors (RF and RI), and

RIN can be ignored.

In this case, the noise gain is defined by the following equation:

Noise Gain = 1 + RF/RI

When the amplifiers transition to Over-The-Top operation with the

input common-mode biased near or above the +VS supply, consider

the value of RIN.

The noise gain of the amplifiers increases as shown in the following

equation:

NoiseGainOTT = 1+ RF

RI RIN+RI RF

1+ RI RFRIN

where Noise GainOTT is the Over-The-Top noise gain.

The dc closed-loop gain remains mostly unaffected (RF/RI). Howev-

er, the loop gain of the amplifiers decreases, as expressed in the

following equation:

AOL1+RFRIto AOL

NoiseGainOTT

Likewise, the closed-loop bandwidth (BWCLOSED_LOOP) of the am-

plifiers changes, going from normal operation to Over-The-Top

operation.

In normal operation,

BWCLOSED_LOOP ≈

GBP

1+ RFRI

In Over-The-Top operation,

BWCLOSED_LOOP ≈

GBP

NoiseGainOTT

Output voltage noise density (eno) is impacted when the device

transitions from normal operation to Over-The-Top operation. Resis-

tor noise is neglected in both modes of operation in the following

equations:

In normal operation, neglecting resistor noise,

eno ≅ en1+ RFRI

where en is input referred voltage noise density.

In Over-The-Top operation, neglecting resistor noise,

eno ≅ en×NoiseGainOTT

OUTPUT

The output of the ADA4099-1 and ADA4099-2 can swing rail-to-rail

to within 45 mV of either supply with no load. The output can source

and sink ~30 mA. The amplifiers are internally compensated to

drive at least 100 pF of load capacitance (CL). Adding a series

resistance of 50 Ω between the output and larger capacitive loads

extends the capacitive drive capability of the amplifiers.

If the ADA4099-1 and ADA4099-2 enter shutdown, the VOUT pin

appears as high impedance with two steering diodes connected to

either supply. In this state, the output typically leaks <5 nA.

SHUTDOWN PINS

The ADA4099-1 and ADA4099-2 have dedicated shutdown pins

(SHDN for the ADA4099-1, and SHDN1 and SHDN2 for the

ADA4099-2 10-lead LFCSP) to place the amplifiers in a very low

power shutdown state when asserted high. A logic high is defined

by a voltage ≥1.5 V applied to SHDN and SHDNx with respect to

the −VS pin. In shutdown, the amplifiers draw <15 μA of supply

current (see Figure 59) and the VOUT pin is placed in a high

impedance state.

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