AD1556AS datasheet(17/24 Pages) AD

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REV. B

AD1555/AD1556

–17–

external stresses such as lightning, the inputs AIN are specifi-

cally designed to ease the design. The external voltage spike

is generally clamped by devices T1 and T2 at about hundred

volts (for instance, devices T1 and T2 can be gas discharge

tubes) and then generates a pulsed current in the serial

resistances (R1, R3, and R2, R4). The AD1555 AIN inputs,

using robust internal clamping diodes to the analog supply

rails, can handle this huge pulsed input current (1.5 A during

s) without experiencing any destructive damages or

latch-up, whether or not the AD1555 is powered on. Mean-

while, enough time should be left between multiple spikes

to avoid excessive power dissipation.

Programming the AD1555

The different hardware events of the AD1555 as multiplexer

inputs selection, programmable gain settings, and power-down

modes are selectable using the control pins bus CB0 to CB4

according to the Table III. This table is only valid when MCLK

is toggling; otherwise, the AD1555 is powered down. When

used in combination with the AD1556, this control bus could

either be loaded by hardware (H/

S pin high) or via the serial

interface of the AD1556 (H/

S pin low).

The multiplexer, which exhibits a break-before-make switching

action, allows various combinations.

AIN (+)

AIN (–)

TIN (+)

TIN (–)

S1(+)

S1(–)

S2(+)

S2(–)

S3(+)

S3(–)

S4(+)

S4(–)

REFIN

REFCAP2

AGND3

22.5k

7.5k

500

AD1555

100

Figure 8. Simplified AD1555 Input Multiplexer

When the ground input is selected, S3(+) and S3(–) are closed,

all the other switches are opened, and the inputs of the pro-

grammable gain amplifier are shorted through an accurate

internal 1 k

Ω resistor. This combination allows accurate calibra-

tion of the offset of the AD1555 for each gain setting. Also, a

system noise calibration can be done using the internal 1 k

Ω

resistor as a noise reference.

Table III. PGA Input and Gain Control

CB4

CB3

CB2

CB1

CB0

Description

0Ground Input with PGA Gain of 1

1Ground Input with PGA Gain of 2.5

0Ground Input with PGA Gain of 8.5

1Ground Input with PGA Gain of 34

0Ground Input with PGA Gain of 128

0Test Inputs TIN(+) and TIN(–) with PGA Gain of 1

1Test Inputs TIN(+) and TIN(–) with PGA Gain of 2.5

0Test Inputs TIN(+) and TIN(–) with PGA Gain of 8.5

1Test Inputs TIN(+) and TIN(–) with PGA Gain of 34

0Test Inputs TIN(+) and TIN(–) with PGA Gain of 128

Signal Inputs AIN(+) and AIN(–) with PGA Gain of 1

Signal Inputs AIN(+) and AIN(–) with PGA Gain of 2.5

Signal Inputs AIN(+) and AIN(–) with PGA Gain of 8.5

Signal Inputs AIN(+) and AIN(–) with PGA Gain of 34

Signal Inputs AIN(+) and AIN(–) with PGA Gain of 128

VREF Input with PGA Gain of 1

Sensor Test 1: Signal inputs AIN(+) and AIN(–) with

AIN(+) and AIN(–) inputs tied respectively to TIN(+)

and TIN(–) inputs and with PGA Gain of 1.

Sensor Test 2: Signal inputs TIN(+) and TIN(–) with

AIN(–) input tied to TIN(–) input and with PGA Gain of 1.

1PGA Powered Down

XChip Powered Down

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