ADA4098-1BUJZ-R5 datasheet(22/33 Pages) AD

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Data Sheet

ADA4098-1/ADA4098-2

THEORY OF OPERATION

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Rev. B | 22 of 33

noise. The singular benefit of this configuration is an increase in

closed-loop amplifier stability.

In normal mode (−VS < VCM < +VS −1.25 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 ampli-

fiers changes going from normal operation to Over-The-Top opera-

tion.

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 devices

transition 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 ADA4098-1 and ADA4098-2 can swing rail-to-rail

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

24 mA and sink 35 mA. The amplifiers are internally compensated

to drive at least 200 pF of CLOAD. Adding a series resistance of

50 Ω between the output and larger capacitive loads extends the

capacitive drive capability of the amplifiers.

If the ADA4098-1 and ADA4098-2 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 (SHDN AND SHDNX)

The ADA4098-1 and ADA4098-2 have dedicated shutdown pins

(SHDN for the ADA4098-1, and SHDN1 and SHDN2 for the

ADA4098-2 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 the SHDN pin and SHDNx pins with

respect to the −VS pin. In shutdown, the amplifiers draw <12 μA of

supply current (see Figure 9) and the VOUT pin is placed in a high

impedance state.

The SHDN pin and SHDNx pins can be driven beyond the +VS

supply up to the absolute maximum voltage (60 V with respect to

−VS) and draw little current (<1.5 μA). For normal active amplifier

operation, the SHDN pin and SHDNx pins can be floated or driven

by an external low voltage source (within 0.5 V of −VS). If the

SHDN pin and SHDNx pins are left floating, an internal current

source (~600 nA) pulls the SHDN pin and SHDNx pins to –VS,

which places the amplifiers into a default, active amplifying state.

Because of the close proximity of the −IN pin (ADA4098-1) and

−INx pins (ADA4098-2 10-lead LFCSP) and the SHDN pin and

SHDNx pins, respectively, fast edges on the −IN pin and −INx

pins may ac-couple to the adjacent high impedance SHDN pin and

SHDNx pins, inadvertently placing the devices in shutdown. If this

scenario is a concern, add a 1 nF capacitor between the SHDN pin

and SHDNx pins and the −VS pin.

Alternatively, the amplifiers can be effectively placed in a low power

state by removing +VS. In this low power state, the inputs typically

leak <1 nA with either the ±IN pins (ADA4098-1) or ±INx pins

(ADA4098-2 10-lead LFCSP) biased between −VS and 70 V above

−VS. If the ±IN pins and ±INx pins are taken below −VS, they

appear as a diode connected to the −VS supply in series with a

resistance of 880 Ω. In this condition, limit the current to <20 mA.

Using an external source to drive the output beyond either ±VS

supply under shutdown conditions may produce unlimited current

and may damage the devices.

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