AD9240
REV.
–15–
If the application requires the largest single-ended input range
(i.e., 0 V to 5 V) of the AD9240, the op amp will require larger
supplies to drive it. Various high speed amplifiers in the Op
Amp Selection Guide of this data sheet can be selected to
accommodate a wide range of supply options. Once again,
clamping the output of the amplifier should be considered for
these applications. Alternatively, a single-ended to differential
op amp driver circuit using the AD8042 could be used to
achieve the 5 V input span while operating from a single +5 V
supply as discussed in the previous section.
Two dc coupled op amp circuits using a noninverting and
inverting topology are discussed below. Although not shown,
the noninverting and inverting topologies can be easily config-
ured as part of an antialiasing filter by using a Sallen-Key or
Multiple-Feedback topology, respectively. An additional R-C
network can be inserted between the op amp’s output and the
AD9240 input to provide a real pole.
Simple Op Amp Buffer
In the simplest case, the input signal to the AD9240 will already
be biased at levels in accordance with the selected input range.
It is simply necessary to provide an adequately low source im-
pedance for the VINA and VINB analog input pins of the A/D.
Figure 35 shows the recommended configuration for a single-
ended drive using an op amp. In this case, the op amp is shown
in a noninverting unity gain configuration driving the VINA pin.
The internal reference drives the VINB pin. Note that the addi-
tion of a small series resistor of 30 Ω to 50 Ω connected to VINA
and VINB will be beneficial in nearly all cases. Refer to the
Analog Input Operation section for a discussion on resistor
selection. Figure 35 shows the proper connection for a 0 V to
5 V input range. Alternative single ended input ranges of 0 V to
2 × VREF can also be realized with the proper configuration of
VREF (refer to the section, Using the Internal Reference).
10mF
VINA
VINB
SENSE
AD9240
0.1mF
R
S
+V
–V
R
S
VREF
5V
0V
U1
2.5V
Figure 35. Single-Ended AD9240 Op Amp Drive Circuit
Op Amp with DC Level-Shifting
Figure 36 shows a dc-coupled level-shifting circuit employing an
op amp, A1, to sum the input signal with the desired dc offset.
Configuring the op amp in the inverting mode with the given
resistor values results in an ac signal gain of –1. If the signal
inversion is undesirable, interchange the VINA and VINB con-
nections to reestablish the original signal polarity. The dc volt-
age at VREF sets the common-mode voltage of the AD9240. For
example, when VREF = 2.5 V, the output level from the op amp
will also be centered around 2.5 V. The use of ratio matched,
thin-film resistor networks will minimize gain and offset errors.
An optional pull-up resistor, R
P
, may also be used to reduce the
output load on VREF to ±1 mA.
0V
DC
+VREF
–VREF
VINA
VINB
AD9240
0.1mF
500V*
0.1mF
500V*
A1
NC
NC
+V
CC
500V*
R
S
VREF
500V*
R
S
R
P
**
AVDD
*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D
**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE
Figure 36. Single-Ended Input With DC-Coupled Level-Shift
AC COUPLING AND INTERFACE ISSUES
For applications where ac coupling is appropriate, the op amp’s
output can be easily level-shifted to the common-mode voltage,
V
CM
, of the AD9240 via a coupling capacitor. This has the
advantage of allowing the op amps common-mode level to be
symmetrically biased to its midsupply level (i.e., (V
CC
+ V
EE
)/
2). Op amps that operate symmetrically with respect to their
power supplies typically provide the best ac performance as well
as greatest input/output span. Hence, various high speed/
performance amplifiers that are restricted to +5 V/–5 V op-
eration and/or specified for +5 V single-supply operation can be
easily configured for the 5 V or 2 V input span of the AD9240,
respectively. The best ac distortion performance is achieved
when the A/D is configured for a 2 V input span and common-
mode voltage of 2.5 V. Note that differential transformer
coupling, which is another form of ac coupling, should be
considered for optimum ac performance.
Simple AC Interface
Figure 37 shows a typical example of an ac-coupled, single-
ended configuration. The bias voltage shifts the bipolar,
ground-referenced input signal to approximately VREF. The
value for C1 and C2
will depend on the size of the resistor, R.
The capacitors, C1 and C2, are typically a 0.1 µF ceramic and
10 µF tantalum capacitor in parallel to achieve a low cutoff
frequency while maintaining a low impedance over a wide fre-
quency range. The combination of the capacitor and the resistor
form a high-pass filter with a high-pass –3 dB frequency deter-
mined by the equation,
f
–3 dB
= 1/(2 × π × R × (C1 + C2))
C2
VINA
VINB
SENSE
AD9240
C1
R
+5V
–5V
R
S
VREF
+VREF
0V
–VREF
V
IN
C2
C1
R
S
Figure 37. AC-Coupled Input
The low impedance VREF voltage source biases both the VINB
input and provides the bias voltage for the VINA input. Figure
37 shows the VREF configured for 2.5 V. Thus the input range
of the A/D is 0 V to 5 V. Other input ranges could be selected
by changing VREF but the A/D’s distortion performance will
