Datasheet

AD7626 Data Sheet
Rev. B | Page 18 of 28
DRIVING THE AD7626
Differential Analog Input Source
Figure 33 shows an ADA4899-1 driving each differential input
to the AD7626.
Single-Ended-to-Differential Driver
For applications using unipolar analog signals, a single-
ended-to-differential driver (as shown in Figure 32) allows
for a differential input into the part. This configuration, when
provided with an input signal of 0 V to 4.096 V, produces a
differential ±4.096 V with midscale at 2.048 V. The one-pole
filter using R = 20  and C = 56 pF provides a corner frequency
of 140 MHz. The VCM output of the AD7626 can be buffered
and then used to provide the required 2.048 V common-mode
voltage.
Single-Ended or Fully Differential High Frequency Driver
In applications that require higher input frequency tones, the
ADA4932-1 can be used to drive the inputs to the AD7626. The
ADA4932-1 is a differential driver, which also allows the user
the option of single-ended-to-differential conversion.
Figure 34 shows the typical circuit for a 50 Ω source impedance
(ac-coupled in this example). The input to the ADA4932-1 is
configured to be balanced to the source impedance (in this case
50 Ω). Further information on balancing the input impedance
to the source impedance can be found on the ADA4932-1
datasheet. The circuit shown in Figure 34 operates with an
overall gain of ~0.5 when the termination input termination
is taken into account.
Alternatively, the ADA4932-1 can be used with a fully diffe-
rential source—it acts as an inverting differential driver.
56pF
20Ω
20Ω
50Ω
IN+
IN–
AD7626
VCM
V–
V+
AD8031, AD8032
U2
U1
ADA4899-1
ADA4899-1
56pF
100nF
100nF
590Ω
590Ω
ANALOG INPUT
(UNIPOLAR 0V TO 4.096V)
0
7648-033
Figure 32. Single-Ended-to-Differential Driver Circuit Using ADA4899-1
56pF
20Ω
–V
S
0V TO V
REF
+V
S
56pF
20Ω
–V
S
V
REF
TO 0V
+V
S
C
REF
10µF
2
REF
1
REF
GND
IN+
IN–
AD7626
ADA4899-1
1
SEE THE VOLTAGE REFERENCE OPTIONS SECTION. CONNECTION TO EXTERNAL REFERENCE SIGNALS
IS DEPENDENT ON THE EN1 AND EN0 SETTINGS.
2
C
REF
IS USUALLY A 10µF CERAMIC CAPACITOR WITH LOW ESL AND ESR.
DECOUPLE REF AND REFIN PINS AS PER THE EN1 AND EN0 RECOMMENDATIONS
VCM
–V
S
+V
S
BUFFERED VCM PIN OUTPUT
GIVES THE REQUIRED 2.048V
COMMON-MODE SUPPLY FOR
ANALOG INPUTS.
VCM
2.048V
AD8031, AD8032
ADA4899-1
REFIN
REF
1
C
REF
10µF
2
0.1µF
07648-025
Figure 33. Driving the AD7626 from a Differential Analog Source Using ADA4899-1