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AD9225

–13–

10 F

VINA

VINB

SENSE

AD9225

0.1 F

–V

VREF

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.

0VDC

+VREF

–VREF

VINA

VINB

AD9225

0.1 F

500 *

0.1 F

500 *

+VCC

500 *

VREF

500 *

RP**

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., (VCC + VEE)/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

+2V

–2V

VIN

10 F

AD9631

0.1 F

10 F

0.1 F

0.5

2.5

4.5

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 k

W) 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.

RS 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 –3dB

p¥6K +(C1+C2)

VINA

VINB

AD9225

VIN

0.1 F

VCM

0.1 F

10 F

0.1 F

Figure 11. AC-Coupled Input-Flexible Input Span,

VCM = 2.5 V

Rev. C

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