AD8555ACP-R2 datasheet(27/28 Pages) AD

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AD8555

Rev. 0 | Page 27 of 28

The bridge circuit with a sensitivity of 2 mV/V is excited by a

5 V supply. The full-scale output voltage from the bridge

(±10 mV) therefore has a common-mode level of 2.5 V. The

AD8555 removes the common-mode component and amplifies

the input signal by a factor of 200 (G1 = 4, G2 = 50, Offset =

128). This results in an output signal of ±2.0 V. In order to pre-

vent this signal from running into the AD8555’s ground rail, the

output offset voltage has to be raised to 2.5 V. This signal is

within the input voltage range of the ADC.

USING THE AD8555 WITH CAPACITIVE SENSORS

Figure 61 shows a crude way of using the AD8555 with capaci-

tive sensors. RP1 and RP2 are resistors implementing a potential

divider to bias VNEG to VDD/2. Recommended values range

from 1 kΩ to 1 MΩ. CS is the capacitive sensor, and RS is a shunt

resistor used to prevent leakage currents from integrating on

the sensor. The value of RS is application specific.

Note that although VNEG is tied to a dc voltage, the only

impedance across the capacitive sensor is RS. Therefore, the only

way for charge to leak away from CS is through RS, assuming the

input bias currents at VPOS and VNEG are negligible.

RP2

RP1

AD8555

VOUT

VDD

VPOS

VNEG

Figure 61. Crude Way of Using the AD8555 with Capacitive Sensors

The weakness of the circuit in Figure 61 is that the AD8555

input bias current at VPOS flows into RS and creates a differen-

tial offset voltage between VPOS and VNEG. This differential

offset voltage is amplified by the AD8555. The input bias cur-

rent at VNEG, on the other hand, flows into RP1 and create a

common-mode shift. This has little impact on VOUT. Despite

this weakness, the arrangement in Figure 61 should work if the

user wants to minimize the number of components around the

sensor, and if the error introduced by the input bias current at

VPOS is considered negligible.

If greater accuracy is needed, the circuit in Figure 62 is recom-

mended. RP1, RP2, and CS are the same as in Figure 61; RP1 and

RP2 should be between 1 kΩ to 1 MΩ. RS in Figure 61 has been

split into two resistors, RS1 and RS2, in Figure 62. Again, the only

way for the capacitive sensor to discharge is through (RS1 + RS2).

The input bias current at VPOS flows through RS2 and RP1, and

the input bias current at VNEG flows through RS1 and RP1. If RS1

is made equal to RS2 and if the input bias currents are equal, the

input bias currents give a common-mode shift at VPOS and

VNEG with no differential offset. This common-mode shift is

attenuated by the AD8555 common-mode rejection. Further-

more, changes in input bias current, e.g., with temperature,

manifest as an input common-mode change, also rejected by the

AD8555.

RS2

RP2

RS1

RP1

AD8555

VOUT

VDD

VPOS

VNEG

Figure 62. Recommended Way of Using the AD8555 with Capacitive Sensors

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