ADA4899-1YCPZ-R7 datasheet(13/20 Pages) AD

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ADA4899-1

Rev. A | Page 13 of 20

THEORY OF OPERATION

The ADA4899-1 is a voltage feedback op amp that combines

unity gain stability with a 1 nV/√Hz input noise. It employs a

highly linear input stage that can maintain greater than −80 dBc

(@ 2 V p-p) distortion out to 10 MHz while in a unity gain

configuration. This rare combination of low gain stability,

input referred noise, and extremely low distortion is the result

of Analog Devices proprietary op amp architecture and high

speed complementary bipolar processing technology.

The simplified ADA4899-1 topology, shown in Figure 45, is a

single gain stage with a unity gain output buffer. It has over

80 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

VOUT

Figure 45. ADA4899-1 Topology

A pair of internally connected diodes limits the differential

voltage between the noninverting input and the inverting input

of the ADA4899-1. Each set of diodes has two series diodes,

which are connected in antiparallel. This limits the differential

voltage between the inputs to approximately ±1.2 V. All of the

ADA4899-1 pins are ESD protected with voltage-limiting diodes

connected between both rails. The protection diodes can handle

10 mA. Currents should be limited through these diodes to 10 mA

or less by using a series limiting resistor.

PACKAGING INNOVATION

The ADA4899-1 is available in both a SOIC and a LFCSP, each

of which has a thermal pad that allows the device to run cooler,

thereby increasing reliability. To help avoid routing around this

pad in board layout, both packages have an extra output pin on

the opposite side of the packages for ease in connecting a feedback

network to the inputs. The secondary output pin also isolates

the interaction of any capacitive load on the output and the self-

inductance of the package and bond wire from the feedback

loop. While using the secondary output for feedback, inductance in

the primary output helps to isolate capacitive loads from the

output impedance of the amplifier.

Both the SOIC and LFCSP have modified pinouts to improve

heavy load second harmonic distortion performance. The intent

of both is to isolate the negative supply pin from the noninverting

input. The LFCSP accomplishes this by rotating the standard

8-lead package pinout counterclockwise by one pin. This puts

the supply pins and output pins on one side of the package and

the input pins on the other. The SOIC is slightly different with

the intent of both isolating the inputs from the supply pins and

giving the user the option of using the ADA4899-1 in a

standard SOIC board layout with little or no modification.

Taking the unused Pin 5 and making it a second negative supply

pin allows for both an input isolated layout and a traditional

layout to be supported.

DISABLE PIN

A three-state input pin is provided on the ADA4899-1 for a

high impedance disable and an optional input bias current

cancellation circuit. The high impedance output allows several

ADA4899-1s to drive the same ADC or output line time-

interleaved. Pulling the DISABLE pin low activates the high

impedance state. See Table 7 for threshold levels. When the

DISABLE pin is left floating (open), the ADA4899-1 operates

normally. With the DISABLE pin pulled within 0.7 V of the

positive supply, an optional input bias current cancellation

circuit is turned on, which lowers the input bias current to less

than 200 nA. In this mode, the user can drive the ADA4899-1

from a high dc source impedance and still maintain minimal

output-referred offset without having to use impedance

matching techniques. In addition, the ADA4899-1 can be

ac-coupled while setting the bias point on the input with a high

dc impedance network. The input bias current cancellation

circuit doubles the input referred current noise, but this effect is

minimal as long as the wideband impedances are kept low (see

Figure 16).

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