AD8553 datasheet(15/20 Pages) AD

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Preliminary Technical Data

AD8426

Rev. PrD | Page 15 of 20

APPLICATIONS INFORMATION

DIFFERENTIAL DRIVE

Figure 14 shows how to configure the AD8426 for differential

output.

+IN

–IN

REF

AD8426

VBIAS

–

OP AMP

+OUT

–OUT

RECOMMENDED OP AMPS: AD8515, AD8641, AD820.

RECOMMENDED R VALUES: 5kΩ to 20kΩ.

Figure 14. Differential Output Using an Op Amp

The differential output is set by the following equation:

VDIFF_OUT = VOUT+ − VOUT− = Gain × (VIN+ − VIN−)

The common-mode output is set by the following equation:

VCM_OUT = (VOUT+ − VOUT−)/2 = VBIAS

The advantage of this circuit is that the dc differential accuracy

depends on the AD8426 and not on the op amp or the resistors.

This circuit takes advantage of the precise control that the AD8426

has of its output voltage relative to the reference voltage. Op amp

dc performance and resistor matching do affect the dc common-

mode output accuracy. However, because common-mode errors

are likely to be rejected by the next device in the signal chain,

these errors typically have little effect on overall system accuracy.

Tips for Best Differential Output Performance

For best ac performance, an op amp with at least 2 MHz gain

bandwidth and 1 V/μs slew rate is recommended. Good choices

for op amps are the AD8641, AD8515, or AD820.

Keep trace lengths from resistors to the inverting terminal of

the op amp as short as possible. Excessive capacitance at this

node can cause the circuit to be unstable. If capacitance cannot

be avoided, use lower value resistors.

For best linearity and ac performance, a minimum positive

supply voltage (+VS) is required. Table 9 shows the minimum

supply voltage required for optimum performance where

VCM_MAX indicates the maximum common-mode voltage

expected at the input of the AD8426.

Table 9. Minimum Positive Supply Voltage

Temperature

Equation

Less than −10°C

+VS > (VCM_MAX + VBIAS)/2 + 1.4 V

−10°C to +25°C

+VS > (VCM_MAX + VBIAS)/2 + 1.25 V

More than +25°C

+VS > (VCM_MAX + VBIAS)/2 + 1.1 V

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