ad1555xp Analog Devices, Inc., ad1555xp Datasheet - Page 17

• Manufacturer Part Number: ad1555xp
• Description: 24-bit Adc With Low Noise Pga
• Manufacturer: Analog Devices, Inc.

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
2 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.
CB4
0
0
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
X
X
REV. B
CB3
0
0
0
0
0
1
1
1
1
1
0
0
0
0
0
1
1
1
X
X
CB2
0
0
0
0
1
0
0
0
0
1
0
0
0
0
1
0
0
0
1
1
CB1
0
0
1
1
0
0
0
1
1
0
0
0
1
1
0
0
0
1
0
1
Table III. PGA Input and Gain Control
CB0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
0
0
1
0
1
X
–17–
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 kΩ 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 kΩ
resistor as a noise reference.
Description
Ground Input with PGA Gain of 1
Ground Input with PGA Gain of 2.5
Ground Input with PGA Gain of 8.5
Ground Input with PGA Gain of 34
Ground Input with PGA Gain of 128
Test Inputs TIN(+) and TIN(–) with PGA Gain of 1
Test Inputs TIN(+) and TIN(–) with PGA Gain of 2.5
Test Inputs TIN(+) and TIN(–) with PGA Gain of 8.5
Test Inputs TIN(+) and TIN(–) with PGA Gain of 34
Test 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
V
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.
PGA Powered Down
Chip Powered Down
REF
Figure 8. Simplified AD1555 Input Multiplexer
Input with PGA Gain of 1
REFCAP2
AGND3
AIN (+)
AIN (–)
TIN (+)
TIN (–)
REFIN
22.5k
7.5k
100
100
50
50
500
500
S1(+)
S1(–)
S2(+)
S2(–)
S3(+)
S3(–)
S4(+)
S4(–)
AD1555/AD1556
AD1555