AD8563ARMZ-R2 datasheet(7/15 Pages) AD

No de pièce	AD8563ARMZ-R2

Description	1.8 V to 5 V Auto-Zero, In-Amp with Shutdown

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Fabricant	AD [Analog Devices]

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Preliminary Technical Data

AD8563

Rev. PrA | Page 7 of 15

APPLICATIONS

GAIN SELECTION (GAIN-SETTING RESISTORS)

The gain of the AD8563 is set according to

G = 2 × (R2/R1)

(1)

Table 5 lists the recommended resistor values. Resistor R1 must

be at least 3.92 kΩ for proper operation. The use of resistors

larger than the recommended values results in higher offset and

higher noise.

Gain accuracy depends on the matching of R1 and R2. Any

mismatch in resistor values results in a gain error. Resistor

value errors due to drift will affect gain by the amount indicated

by Equation 1. However, due to the current-mode operation of

the AD8563, a mismatch in R1 and R2 does not degrade the

CMR.

Take care when selecting and positioning the gain setting

resistors. The resistors should be made of the same material and

package style. Surface-mount resistors are recommended. They

should be positioned as close together

as possible to minimize TC errors.

To maintain good CMR vs. frequency, the parasitic capacitance

on the R1 gain setting pins should be minimized and matched.

This also helps maintain a low gain error at G < 10.

If resistor trimming is required to set a precise gain, trim

Resistor R2 only. Using a potentiometer for R1 degrades the

amplifier’s performance.

REFERENCE CONNECTION

Unlike traditional three op amp instrumentation amplifiers,

parasitic resistance in series with VREF (Pin 7) does not degrade

CMR performance. This allows the AD8563 to attain its extremely

high CMR performance without the use of an external buffer

amplifier to drive the VREF pin, which is required by industry-

standard instrumentation amplifiers. This helps save valuable

printed circuit board space and minimizes system costs.

For optimal performance in single-supply applications, VREF

should be set with a low noise precision voltage reference.

However, for a lower system cost, the reference voltage can be

set with a simple resistor voltage divider between the supply and

ground (see Figure 3). This configuration results in degraded

output offset performance if the resistors deviate from their

ideal values. In dual-supply applications, VREF can be connected

to ground.

The VREF pin current is approximately 20 pA, and as a result, an

external buffer is not required.

DISABLE FUNCTION

The AD8563 provides a shutdown function to conserve power

when the device is not needed. Although there is a 1 μA pull-up

current on the ENABLE pin, Pin 6 should be connected to the

positive supply for normal operation and to the negative supply

to turn the device off. It is not recommended to leave Pin 6

floating.

Turn-on time upon switching Pin 6 high is dominated by the

output filters. When the device is disabled, the output becomes

high impedance, enabling a multiplexing application of multiple

AD8563 instrumentation amplifiers.

OUTPUT FILTERING

Filter Capacitor C2 is required to limit the amount of switching

noise present at the output. The recommended bandwidth of

the filter created by C2 and R2 is 1.4 kHz. The user should first

select R1 and R2 based on the desired gain, then select C2 based on

C2 = 1/(1400 × 2 × π × R2)

(2)

Addition of another single-pole RC filter of 1.4 kHz on the

output (R3 and C3 in Figure 3 to Figure 5) is required for

bandwidths greater than 10 Hz. These two filters produce an

overall bandwidth of 1 kHz.

When driving an ADC, the recommended values for the second

filter are R3 = 100 Ω and C3 = 1 μF. This filter is required to

achieve the specified performance. It also acts as an anti-

aliasing filter for the ADC. If a sampling ADC is not being

driven, the value of the capacitor can be reduced, but the filter

frequency should remain unchanged.

For applications with low bandwidths (<10 Hz), only the first

filter is required. In this case, the high frequency noise from the

auto-zero amplifier (output amplifier) is not filtered before the

following stage.

CLOCK FEEDTHROUGH

The AD8563 uses two synchronized clocks to perform the auto-

correction. The input voltage-to-current amplifiers are

corrected at 60 kHz.

Trace amounts of these clock frequencies can be observed at the

output. The amount of feedthrough is dependent upon the gain,

because the auto-correction noise has an input and output

referred term. The correction feedthrough is also dependent

upon the values of the external filters R2/C2, and R3/C3.

LOW IMPEDANCE OUTPUT

For applications where a low output impedance is required, the

circuit in Figure 5 should be used. This provides the same

filtering performance as shown in the configuration in Figure 6.

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