ada4932 Analog Devices, Inc., ada4932 Datasheet - Page 25

• Manufacturer Part Number: ada4932
• Description: Low Power Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

HIGH PERFORMANCE ADC DRIVING
The ADA4932-x is ideally suited for broadband dc-coupled
applications. The circuit in Figure 67 shows a front-end
connection for an ADA4932-1 driving an AD9245, a 14-bit,
20 MSPS/40 MSPS/65 MSPS/80 MSPS ADC, with dc coupling
on the ADA4932-1 input and output. (The AD9245 achieves
its optimum performance when driven differentially.) The
ADA4932-1 eliminates the need for a transformer to drive the
ADC and performs a single-ended-to-differential conversion and
buffering of the driving signal.
The ADA4932-1 is configured with a single 3.3 V supply and a
gain of 1 for a single-ended input to differential output. The
53.6 Ω termination resistor, in parallel with the single-ended
input impedance of approximately 665 Ω, provides a 50 Ω
termination for the source. The additional 25.5 Ω (524.5 Ω
total) at the inverting input balances the parallel impedance
of the 50 Ω source and the termination resistor driving the
noninverting input.
SIGNAL
GENERATOR
50Ω
2V p-p
1V p-p CENTERED
Figure 67. ADA4932-1 Driving an AD9245 ADC with DC-Coupled Input and Output
AT GROUND
53.6Ω
0.1µF
10kΩ
10kΩ
1%
1%
25.5Ω
499Ω
499Ω
V
OCM
Rev. 0 | Page 25 of 28
ADA4932-1
511Ω
511Ω
3.3V
0.1µF
In this example, the signal generator has a 1 V p-p symmetric,
ground-referenced bipolar output when terminated in 50 Ω.
The V
with 1% resistors to maximize output dynamic range on the
tight 3.3 V supply.
Because the inputs are dc-coupled, dc common-mode current
flows in the feedback loops, and a nominal dc level of 0.84 V is
present at the amplifier input terminals. A fraction of the output
signal is also present at the input terminals as a common-mode
signal; its level is equal to the ac output swing at the noninverting
output, divided down by the feedback factor of the lower loop.
In this example, this ripple is 0.5 V p-p × [524.5/(524.5 + 511)] =
0.25 V p-p. This ac signal is riding on the 0.84 V dc level, produc-
ing a voltage swing between 0.72 V and 0.97 V at the input
terminals. This is well within the specified limits of 0.2 V to 1.5 V.
With an output common-mode voltage of 1.65 V, each ADA4932-1
output swings between 1.4 V and 1.9 V, opposite in phase, provid-
ing a gain of 1 and a 1 V p-p differential signal to the ADC input.
The differential RC section between the ADA4932-1 output and
the ADC provides single-pole low-pass filtering and extra buffering
for the current spikes that are output from the ADC input when its
SHA capacitors are discharged.
The AD9245 is configured for a 1 V p-p full-scale input by
connecting its SENSE pin to VREF, as shown in Figure 67.
33Ω
33Ω
20pF
OCM
0.1µF
V
V
OUT, dm
input is bypassed for noise reduction, and set externally
OUT, cm
10µF
= 1V p-p
VIN–
VIN+ VREF SENSE AGND
= 1.65V
+
AD9245
AVDD
ADA4932-1/ADA4932-2
0.1µF