AN-1026
APPLICATION NOTE
One Technology Way • P. O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
High Speed Differential ADC Driver Design Considerations
by John Ardizzoni and Jonathan Pearson
For the discussions that follow, some definitions are in order.
If the input signal is balanced, VIP and VIN are nominally equal
in amplitude and opposite in phase with respect to a common
reference voltage. When the input is single-ended, one input is
at a fixed voltage, and the other varies with respect to it. In either
case, the input signal is defined as VIP – VIN.
INTRODUCTION
Most modern high performance ADCs use differential inputs to
reject common-mode noise and interference, increase dynamic
range by a factor of 2, and improve overall performance due to
balanced signaling. Though ADCs with differential inputs can
accept single-ended input signals, optimum ADC performance
is achieved when the input signal is differential. ADC drivers—
circuits often specifically designed to provide such signals—perform
many important functions, including amplitude scaling, single-
ended-to-differential conversion, buffering, common-mode offset
adjustment, and filtering. Since the introduction of the AD8138,
differential ADC drivers have become essential signal conditioning
elements in data acquisition systems.
The differential-mode input voltage, VIN, dm, and the common-
mode input voltage, VIN, cm, are defined in Equation 1 and
Equation 2.
V
IN, dm = VIP – VIN
(1)
VIP +VIN
VIN, cm
=
(2)
2
This common-mode definition is intuitive when applied to
balanced inputs, but it is also valid for single-ended inputs.
R
F1
R
G1
V
V
A
+
V
IP
V
–
ON
The output also has a differential mode and a common mode,
defined in Equation 3 and Equation 4.
V
V
IN, dm
OCM
OUT, dm
–
V
V
+
IN
OP
V
R
A
G2
R
F2
VOUT, dm = VOP – VON
VOP +VON
(3)
Figure 1. Differential Amplifier
VOUT, cm
=
(4)
A basic fully differential voltage-feedback ADC driver is shown
in Figure 1. Two differences from a traditional op-amp feedback
circuit can be seen. The differential ADC driver has an additional
output terminal (VON) and an additional input terminal (VOCM).
These terminals provide great flexibility when interfacing
signals to ADCs that have differential inputs.
2
Note the difference between the actual output common-mode
voltage, VOUT, cm, and the VOCM input terminal, which establishes
the output common-mode level.
The analysis of differential ADC drivers is considerably more
complex than that of traditional op amps. To simplify the algebra, it
is expedient to define two feedback factors, β1 and β2, as given
in Equation 5 and Equation 6.
Instead of a single-ended output, the differential ADC driver
produces a balanced differential output, with respect to VOCM
,
between VOP and VON. (P indicates positive and N indicates
negative.) The VOCM input controls the output common-mode
voltage. As long as the inputs and outputs stay within their
specified limits, the output common-mode voltage must equal
the voltage applied to the VOCM input. Negative feedback and
high open-loop gain cause the voltages at the amplifier input
terminals, VA+ and VA–, to be essentially equal.
RG1
F1 + RG1
β1 =
β2 =
(5)
(6)
R
RG2
F2 + RG2
R
Rev. 0
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rights of third parties that may result from its use. Specifications subject to change without notice. No
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Trademarks and registeredtrademarks arethe property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
Fax: 781.461.3113
www.analog.com
©2009 Analog Devices, Inc. All rights reserved.
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