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ADA4896-2/ADA4897-1

Rev. 0 | Page 18 of 28

THEORY OF OPERATION

AMPLIFIER DESCRIPTION

The ADA4896-2/ADA4897-1 are 1 nV/√Hz input noise

amplifiers that consume 3 mA from supplies ranging from

3 V to 10 V. Utilizing the Analog Devices XFCB3 process, the

bandwidth is in excess of 200 MHz and unity gain stable and

the input structure results in an extremely low input of 1/f noise

for a high speed amplifier. The rail-to-rail output stage is

designed to drive a heavy feedback load required to achieve an

overall low output referred noise. Unlike other low noise unity

gain stable amplifiers, the large signal bandwidth has been

enhanced beyond the typical fundamental limits to meet more

demanding system requirements. The maximum offset of 500 μV

and drift of 1 μV/°C make the ADA4896-2/ADA4897-1 an

excellent amplifier choice even when the noise is not needed

because there is minimal power penalty in achieving the low

input noise or the high bandwidth.

INPUT PROTECTION

The ADA4896-2/ADA4897-1 are fully protected from ESD

events, withstanding human body model ESD events of 2.5 kV

and charge device model events of 1 kV with no measured

performance degradation. The precision input is protected

with an ESD network between the power supplies and diode

clamps across the input device pair, as shown in Figure 43.

ESD

VEE

VCC

BIAS

TO THE REST OF THE AMPLIFIER

ESD

Figure 43. Input Stage and Protection Diodes

For differential voltages above approximately 0.7 V, the diode

clamps start to conduct. Too much current can cause damage

due to excessive heating. If large differential voltages must

be sustained across the input terminals, it is recommended

that the current through the input clamps be limited to below

10 mA. Series input resistors that are sized appropriately for the

expected differential overvoltage provide the needed protection.

The ESD clamps start to conduct for input voltages that are

more than 0.7 V above the positive supply and input voltages

more than 0.7 V below the negative supply. It is recommended

that the fault current be limited to less than 10 mA if an

overvoltage condition is expected.

DISABLE OPERATION

Figure 44 shows the ADA4897-1 power-down circuitry. If the

DISABLE pin is left unconnected, the base of the input PNP

transistor is pulled high through the internal pull-up resistor to

the positive supply and the part is turned on. Pulling the

DISABLE pin to ≥2 V below the positive supply turns the part

off, reducing the supply current to approximately 18 μA for a

5 V voltage supply.

VCC

VEE

DISABLE

ESD

IBIAS

AMPLIFIER

BIAS

Figure 44. DISABLE Circuit

The DISABLE pin is protected with ESD clamps, as shown in

Figure 44. Voltages beyond the power supplies cause these

diodes to conduct. For protection of the DISABLE pin, the

voltage to this pin should not exceed 0.7 V of the supply voltage,

or the input current should be restricted to less than 10 mA

with a series resistor.

When the amplifier is disabled, its output goes to a high

impedance state. The output impedance decreases as frequency

increases; this effect can be observed in Figure 35. In disable

mode, a forward isolation of 50 dB can be achieved at 10 MHz.

Figure 42 shows the forward isolation vs. frequency data.

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