Datasheet

AD9225
–13–
10F
VINA
VINB
SENSE
AD9225
0.1F
R
S
+V
–V
R
S
VREF
4V
0V
U1
2.0V
Figure 8. Single-Ended AD9225 Op Amp Drive Circuit
Op Amp with DC Level Shifting
Figure 9 shows a dc-coupled level shifting circuit employing an op
amp, A1, to sum the input signal with the desired dc set. Configur-
ing 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 undesir-
able, interchange the VINA and VINB connections to re-establish
the original signal polarity. The dc voltage at VREF sets the
common-mode voltage of the AD9225. For example, when
VREF = 2.0 V, the input level from the op amp will also be cen-
tered around 2.0 V. The use of ratio matched, thin-film resistor
networks will minimize gain and offset errors. Also, an optional
pull-up resistor, RP, may be used to reduce the output load on
VREF to less than its 1 mA maximum.
0V
DC
+VREF
–VREF
VINA
VINB
AD9225
0.1F
500*
0.1F
500*
7
1
2
3
4
5
A1
6
NC
NC
+V
CC
500*
R
S
VREF
500*
R
S
R
P
**
+V
*OPTIONAL RESISTOR NETWORK-OHMTEK ORNA500D
**OPTIONAL PULL-UP RESISTOR WHEN USING INTERNAL REFERENCE
Figure 9. Single-Ended Input with DC-Coupled Level Shift
AC COUPLING AND INTERFACE ISSUES
For applications where ac coupling is appropriate, the op amp
output can be easily level-shifted via a coupling capacitor. This has
the advantage of allowing the op amp 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 great-
est input/output span. Various high speed/performance amplifiers
which are restricted to +5 V/–5 V operation and/or specified for
+5 V single-supply operation can be easily configured for the 4 V
or 2 V input span of the AD9225. Note that differential trans-
former coupling, which is another form of ac coupling, should be
considered for optimum ac performance.
Simple AC Interface
Figure 10 shows a typical example of an ac-coupled, single-ended
configuration. The bias voltage shifts the bipolar, ground-refer-
enced input signal to approximately AVDD/2. The value for C1
and C2 will depend on the size of the resistor, R. The capacitors,
C1 and C2, are a 0.1 mF ceramic and 10 mF tantalum capacitor in
parallel to achieve a low cutoff frequency while maintaining a low
impedance over a wide frequency range. The combination of the
capacitor and the resistor form a high-pass filter with a high-pass –
3 dB frequency determined by the equation
f
–3 dB
= 1/(2 ¥ ␲ ¥ R ¥ (C1 + C2))
The low impedance VREF voltage source biases both the VINB
input and provides the bias voltage for the VINA input. Figure 10
shows the VREF configured for 2.0 V thus the input range of the
ADC is 0 V to 4 V. Other input ranges could be selected by chang-
ing VREF.
VINA
VINB
AD9225
+5V
–5V
R
S
0V
+2V
–2V
V
IN
C1
10F
R
S
AD9631
+V
+V
C2
0.1F
10F
0.1F
0.5
2.5
4.5
R
R
R
R
Figure 10. AC-Coupled Input
Alternative AC Interface
Figure 11 shows a flexible ac-coupled circuit that can be config-
ured for different input spans. Since the common-mode voltage of
VINA and VINB are biased to midsupply independent of VREF,
VREF can be pin strapped or reconfigured to achieve input spans
between 2 V and 4 V p-p. The AD9225’s CMRR along with the
symmetrical coupling R-C networks will reject both power supply
variations and noise. The resistors, R, establish the common-mode
voltage. They may have a high value (e.g., 5 kW) to minimize
power consumption and establish a low cutoff frequency. The
capacitors, C1 and C2, are typically a 0.1 mF ceramic and 10 mF
tantalum capacitor in parallel to achieve a low cutoff frequency
while maintaining a low impedance over a wide frequency range.
R
S
isolates the buffer amplifier from the ADC input. The optimum
performance is achieved when VINA and VINB are driven via
symmetrical networks. The f
–3 dB
point can be approximated by
the equation
f
–3 dB
=
1
2 p¥6K +(C1+ C2)
VINA
VINB
AD9225
1k
R
S
V
IN
C2
0.1F
R
S
VCM
1k
C2
0.1F
C1
10F
C1
10F
C3
0.1F
Figure 11. AC-Coupled Input-Flexible Input Span,
V
CM
= 2.5 V
Rev. C