ADA4895-1/ADA4895-2

Data Sheet

THEORY OF OPERATION

AMPLIFIER DESCRIPTION

The ADA4895-1/ADA4895-2 amplifiers have an input noise of

1 nV/√Hz and consume 3 mA per amplifier from supply voltages

of 3 V to 10 V. Using the Analog Devices XFCB3 process, the

ADA4895-1/ADA4895-2 have a gain bandwidth product in

excess of 1.5 GHz and are gain ≥ 10 stable, with an input

structure that results in an extremely low input 1/f noise for a

relatively high speed amplifier.

The rail-to-rail output stage is designed to drive the heavy feedback

load required to achieve an overall low output referred noise.

The low input noise and high bandwidth of the ADA4895-1/

ADA4895-2 are achieved with minimal power penalty. For this

reason, the maximum offset voltage of 350 µV and voltage drift

of 0.15 µV/°C make the ADA4895-1/ADA4895-2 an excellent

choice, even when the low noise performance of the amplifier is

not needed.

For any gain greater than 10, the closed-loop frequency response

of a basic noninverting configuration can be approximated by

)

G

F

G

R

R

R

+

For inverting gain configurations, the source impedance must be

For gains lower than 10, see the Using the ADA4895-1/ADA4895-2

at a Gain < +10 section, or use the ADA4897-1/ADA4897-2, which

is a unity-gain stable amplifier with 230 MHz bandwidth.

INPUT PROTECTION

The ADA4895-1/ADA4895-2 are fully protected from ESD events

and can withstand human body model ESD events of 2.5 kV and

charged-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 44.

+IN

ESD

ESD

BIAS

TO THE REST OF THE AMPLIFIER

–IN

ESD

ESD

Figure 44. Input Stage and Protection Diodes

At differential voltages above approximately 0.7 V, the diode

clamps begin 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 less than 10 mA.

Series input resistors that are sized appropriately for the expected

differential overvoltage provide the needed protection.

The ESD clamps begin to conduct at input voltages that are more

than 0.7 V above the positive supply or more than 0.7 V below

the negative supply. If an overvoltage condition is expected, it is

recommended that the fault current be limited to less than 10 mA.

DISABLE OPERATION

Figure 45 shows the ADA4895-1/ADA4895-2 power-down

circuitry. If the DISABLEx 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 device is turned on. Pulling

the DISABLEx pin more than 2 V below the positive supply turns

the device off, reducing the supply current to approximately

50 µA for a 5 V voltage supply.

DISABLEx

ESD

ESD

TO

AMPLIFIER

BIAS

Figure 45. DISABLEx Circuit

The DISABLEx pin is protected by ESD clamps, as shown in

Figure 45. Voltages beyond the power supplies cause these diodes

to conduct. For protection of the DISABLEx pins, the voltage to

these pins should not exceed 0.7 V beyond the supply voltage, or

the input current should be restricted to less than 10 mA with a

series resistor.

Rev. B | Page 16 of 24