Data Sheet ADA4932-1/ADA4932-2
Rev. E | Page 23 of 27
4. The feedback resistor value is modified as a final gain
adjustment to obtain the desired output voltage.
To make the output voltage VOUT = 1 V p-p, calculate RF by
using the following formula:
509
03.1
5.5241
,
ppV
ppV
V
RRVDesired
R
TH
TS
G
dmOUT
F
The closest standard 1% value to 509 Ω is 511 Ω, which
gives a differential output voltage of 1.00 V p-p.
The final circuit is shown in Figure 62.
ADA4932-1/
ADA4932-2
R
L
V
OUT, dm
1.00V p-p
+V
S
–V
S
R
S
50Ω
R
G
499Ω
R
G
499Ω
R
F
511Ω
R
F
511Ω
V
OCM
V
S
2V p-p
1V p-p
R
T
53.6Ω
R
TS
25.5Ω
07752-054
Figure 62. Terminated Single-Ended-to-Differential System with G = 2
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4932-1/ADA4932-2 input common-mode range is
shifted down by approximately one VBE, in contrast to other
ADC drivers with centered input ranges such as the ADA4939-1/
ADA4939-2. The downward-shifted input common-mode
range is especially suited to dc-coupled, single-ended-to-
differential, and single-supply applications.
For ±5 V operation, the input common-mode range at the
summing nodes of the amplifier is specified as −4.8 V to +3.2 V,
and is specified as +0.2 V to +3.2 V with a +5 V supply. To
avoid nonlinearities, the voltage swing at the +IN and −IN
terminals must be confined to these ranges.
INPUT AND OUTPUT CAPACITIVE AC COUPLING
While the ADA4932-1/ADA4932-2 is best suited to dc-coupled
applications, it is nonetheless possible to use it in ac-coupled
circuits. Input ac coupling capacitors can be inserted between
the source and RG. This ac coupling blocks the flow of the dc
common-mode feedback current and causes the ADA4932-1/
ADA4932-2 dc input common-mode voltage to equal the dc
output common-mode voltage. These ac coupling capacitors must
be placed in both loops to keep the feedback factors matched.
Output ac coupling capacitors can be placed in series between
each output and its respective load.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The VOCM/VOCMx pin of the ADA4932-1/ADA4932-2 is internally
biased with a voltage divider comprised of two 50 kΩ resistors
across the supplies, with a tap at a voltage approximately equal
to the midsupply point, [(+VS) + (−VS)]/2. Because of this
internal divider, the VOCM/VOCMx pin sources and sinks current,
depending on the externally applied voltage and its associated
source resistance. Relying on the internal bias results in an
output common-mode voltage that is within about 100 mV of
the expected value.
In cases where more accurate control of the output common-
mode level is required, it is recommended that an external
source or resistor divider be used with source resistance less
than 100 Ω. If an external voltage divider consisting of equal
resistor values is used to set VOCM to midsupply with greater
accuracy than produced internally, higher values can be used
because the external resistors are placed in parallel with the
internal resistors. The output common-mode offset listed in the
Specifications section assumes that the VOCM input is driven by a
low impedance voltage source.
It is also possible to connect the VOCM input to a common-mode
level (CML) output of an ADC; however, care must be taken to
ensure that the output has sufficient drive capability. The input
impedance of the VOCM/VOCMx pin is approximately 25 kΩ. If
multiple ADA4932-1/ADA4932-2 devices share one ADC
reference output, a buffer may be necessary to drive the parallel
inputs.
HIGH PERFORMANCE PRECISION ADC DRIVER
Using a differential amplifier to drive an ADC successfully is
linked to balancing each side of the differential amplifier
correctly. Figure 64 shows the schematic for the ADA4932-1,
AD7626, and associated circuitry. In the test circuit used, a
2.4 MHz band-pass filter follows the signal source. The band-
pass filter eliminates harmonics of the 2.4 MHz signal and
ensures that only the frequency of interest is passed and
processed by the ADA4932-1 and AD7626.
The ADA4932-1 is particularly useful when driving higher fre-
quency inputs to the AD7626, a 10 MSPS ADC with a switched
capacitor input. The resistor (R8, R9) and capacitor (C5, C6)
circuit between the ADA4932-1 and AD7626 IN+ and IN− pins
acts as a low-pass filter to noise. The filter limits the input band-
width to the AD7626, but its main function is to optimize the
interface between the driving amplifier and the AD7626. The
series resistor isolates the driver amplifier from high frequency
switching spikes from the ADC switched capacitor front end.
The AD7626 data sheet shows values of 20 and 56 pF. In
Figure 64, these values were empirically optimized to 33 and
56 pF. The resistor-capacitor combination can be optimized slightly
for the circuit and input frequency being converted by simply
varying the R-C combination; however, keep in mind that
having the incorrect combination limits the THD and linearity
performance of the AD7626. In addition, increasing the bandwidth
as seen by the ADC introduces more noise.