AD8264ACPZ datasheet(29/40 Pages) AD

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Data Sheet

AD8264

Rev. B | Page 29 of 40

IPPx

VPOS

IPNx

VOLx

VOHx

INTERPOLATOR

PREAMP

6dB (2×)

OFSx

OPPx

GAIN

INTERFACE

100Ω

747Ω

107Ω

100Ω

–

VOCM

COMM

GNHx

GNLO

BIAS

VNEG

NONINVERTING

AMPLIFIER INPUT

INVERTING AMPLIFIER

INPUT (NOT USED)

POWER

SUPPLIES

VGAx

SINGLE-ENDED HS

VGA OUTPUT

DIFFERENTIAL

VGA OUTPUT

OFFSET

ADJUST

OUTPUT COMMON-MODE

VOLTAGE ADJUSTMENT

DIFFERENTIAL GAIN

CONTROL INPUTS

1.2V p-p MAX @ ±2.5V

2V p-p MAX @ ±3.5V TO ±3.3V

3V p-p MAX@ ±5V (PREAMP

DRIVE LIMITED)

2.3nV/√Hz

DIFFERENTIAL OUTPUT NEVER LIMITS

BECAUSE VGA LIMITS FIRST.

DIFFERENTIAL OUTPUT SWING = 2x VGA OUT

5.2V p-p MAX @ ±2.5V

8V p-p MAX @ ±3.5V TO ±3.3V

15V p-p MAX @ ±5V

73nV/√Hz

PREAMP OUTPUT

(NOT USED)

2.6V p-p MAX @ ±2.5V

4V p-p MAX @ ±3.5V TO ±3.3V

7.5V p-p MAX @ ±5V

34nV/√Hz

COMPOSITE GAIN IS +6dB TO +30dB

ATTENUATOR

–24dB TO 0dB

FIXED GAIN VGA

AMPLIFIER

18dB (8×)

DIFFERENTIAL OUTPUT

AMPLIFIER 6dB (2×)

1kΩ

2kΩ

1kΩ

2kΩ

Figure 111. Single-Channel Block Diagram

POST AMPLIFIER

From the preamp input to the VGA output (VGAx), the gain

is noninverting. As can be seen in Figure 111, the VGAx pins

drive the positive input of the differential amplifier. The gain

is inverting from the input of the preamp to the output pin at

VOLx, and the gain is noninverting to the output VOHx.

Other than the input from VGAx, each differential amplifier

has two additional inputs: VOCM and OFSx. A common

VOCM pin is shared among all four postamplifiers, while

separate OFSx pins are provided for each channel.

VOCM Pin

The VOCM pin sets the common-mode voltage of the differential

output and must be biased by an external voltage. When driving

a dc-coupled ADC, the voltage typically comes from the ADC

reference, as shown in the Applications Information section.

If dc level shift is not necessary, the VOCM pin is connected

to ground.

OFSx Pins

The OFSx pins are the inverting inputs of the differential post

amplifiers and can be used to prebias a differential dc offset at

the output. This is very useful when the input is a unipolar pulse

because the user can set up the gain and the offset in such a way

as to optimally map a unipolar pulse into the full-scale input of

an ADC, while dc coupling throughout.

If dc offset is not desired, then connect the OFSx pins to ground.

However, the OFSx pins can also be used as separate inputs if the

user wants this function.

NOISE

At maximum gain, the preamplifier is the primary contributor

of noise and results in a differential output referred noise of

roughly 73 nV/Hz. The noise at the VGAx outputs is 34 nV/Hz,

and because of the gain-of-2, the VGA output noise is amplified

by 6 dB to 68 nV/Hz. The differential amplifier, including the

gain setting resistors, contributes another 26 nV/Hz, and the

rms sum results in a total noise of 73 nV/Hz. At the lowest

gain, the noise at the VGA output is approximately 19 nV/Hz, and

when multiplied by two, it results in 38 nV/Hz at the differential

output; again, rms summing this with the 26 nV/Hz of the

differential amplifier causes the total output referred noise to

be approximately 46 nV/Hz.

The input referred noise to the preamplifier at maximum gain

is 2.3 nV/Hz and increases with decreasing gain. Note that all

noise numbers include the necessary gain setting resistors.

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