Datasheet

AD8264
Rev. A | Page 29 of 40
IPPx
VPOS
IPNx
VOLx
VOHx
INTERPOLATOR
PREAMP
6dB (2×)
OFSx
OPPx
GAIN
INTERFACE
100Ω
747Ω
107Ω
100Ω
+
–
VOCMCOMM
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
1
1
2
2 3
3
07736-081
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 the OFSx pins should be connected
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.