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AD8555
(Rev.:Rev0)

Zero-Drift, Digitally Programmable Sensor Signal Amplifier

Analog Devices 

OPEN WIRE FAULT DETECTION

The inputs to A1 and A2, VNEG and VPOS, each have a com-

parator to detect whether VNEG or VPOS exceeds a threshold

voltage, nominally VDD − 1.1 V. If (VNEG > VDD − 1.1 V) or

(VPOS > VDD − 1.1 V), VOUT is clamped to VSS. The output

current limit circuit is disabled in this mode, but the maximum

sink current is approximately 50 mA when VDD = 5 V. The

inputs to A1 and A2, VNEG and VPOS, are also pulled up to

VDD by currents IP1 and IP2. These are both nominally 18 nA

and matched to within 5 nA. If the inputs to A1 or A2 are acci-

dentally left floating, e.g., an open wire fault, IP1 and IP2 pull

them to VDD, which would cause VOUT to swing to VSS, al-

lowing this fault to be detected. It is not possible to disable IP1

and IP2, nor the clamping of VOUT to VSS, when VNEG or

VPOS approaches VDD.

SHORTED WIRE FAULT DETECTION

The AD8555 provides fault detection, in the case where VPOS,

VNEG, or VCLAMP shorts to VDD and VSS. Figure 50 shows

the voltage regions at VPOS, VNEG, and VCLAMP that trigger

an error condition. When an error condition occurs, the VOUT

pin is shorted to VSS. Table 8 lists the voltage levels shown in

Figure 50.

VPOS

ERROR

VDD

VINH

NORMAL

VNEG

ERROR

VDD

VINH

NORMAL

VCLAMP

VDD

NORMAL

ERROR

VINL

VSS

ERROR

VINL

VSS

ERROR

VCLL

VSS

Figure 50. Voltage Regions at VPOS, VNEG, and VCLAMP

That Trigger a Fault Condition

Table 8. Typical VINL, VINH, and VCLL Values (VDD = 5 V)

Voltage Typical Min Typical Max Purpose

VINH 3.9 V

4.2 V

Short to VDD

Fault Detection

VINL

0.195 V

0.55 V

Short to VSS

Fault Detection

VCLL 1 V

1.2 V

Short to VSS

Fault Detection

AD8555

FLOATING VPOS, VNEG, OR VCLAMP FAULT

DETECTION

A floating fault condition at the VPOS, VNEG, or VCLAMP

pins is detected by using a low current to pull a floating input

into an error voltage range, which is defined in the previous

section. In this way, the VOUT pin is shorted to VSS when a

floating input is detected. Table 9 lists the currents used.

Table 9. Floating Fault Detection at VPOS, VNEG, and

VCLAMP

Pin

Typical Current

Goal of Current

VPOS 16 nA pull-up

Pull VPOS above VINH

VNEG 16 nA pull-up

Pull VNEG above VINH

VCLAMP 0.2 µA pull-down

Pull VCLAMP below VCLL

DEVICE PROGRAMMING

Digital Interface

The digital interface allows the first stage gain, second stage

gain, and output offset to be adjusted and allows desired values

for these parameters to be permanently stored by selectively

blowing polysilicon fuses. To minimize pin count and board

space, a single-wire digital interface is used. The digital input

pin, DIGIN, has hysteresis to minimize the possibility of inad-

vertent triggering with slow signals. It also has a pull-down

current sink to allow it to be left floating when programming is

not being performed. The pull-down ensures inactive status of

the digital input by forcing a dc low voltage on DIGIN.

A short pulse at DIGIN from low to high and back to low again,

e.g., between 50 ns and 10 µs long, loads a 0 into a shift register.

A long pulse at DIGIN, e.g., 50 µs or longer, loads a 1 into the

shift register. The time between pulses should be at least 10 µs.

Assuming VSS = 0 V, voltages at DIGIN between VSS and 0.2 ×

VDD are recognized as a low, and voltages at DIGIN between

0.8 × VDD and VDD are recognized as a high. A timing dia-

gram example showing the waveform for entering code 010011

into the shift register is shown in Figure 51.

Rev. 0 | Page 19 of 28

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