REV. B

AD1555/AD1556

–10–

AD1555 PIN FUNCTION DESCRIPTIONS

Pin No.

Mnemonic

Description

1

AGND1

Analog Ground

2

PGAOUT

Programmable Gain Amplifier Output. The output of the on-chip programmable gain amplifier is

available at this pin. Refer to Table III for PGA gain settings selection.

3, 26

Positive Analog Supply Voltage. +5 V nominal.

4, 20, 21

Negative Analog Supply Voltage. –5 V nominal.

5

AIN(+)

Mux Input. Noninverting signal to the PGA mux input. Refer to Table III for input selection.

6

AIN(–)

Mux Input. Inverting signal to the PGA mux input. Refer to Table III for input selection.

7

TIN(+)

Mux Input. Noninverting test signal to the PGA mux input. Refer to Table III for input selection.

8

TIN(–)

Mux Input. Inverting test signal to the PGA mux input. Refer to Table III for input selection.

9NCPin for Factory Use Only. This pin must be kept not connected for normal operation.

10–14

CB0–CB4

Modulator Control. These input pins control the mux selection, the PGA gain settings, and the

standby modes of the AD1555. When used with the AD1556, these pins are generally directly tied

to the CB0–CB4 output pins of the AD1556. CB0–CB2 are generally used to set the PGA gain or

cause it to enter in the PGA standby mode (refer to Table III). CB3 and CB4 select the mux input

voltage applied to the PGA as described in Table III.

15

MFLG

Modulator Error. Digital output that is pulsed high if an overrange condition occurs in the modulator.

16

DGND

Digital Ground

17

MDATA

Modulator Output. The bitstream generated by the modulator is output in a return-to-zero data

format. The data is valid for approximately one-half a MCLK cycle. Refer to Figure 3.

18

MCLK

Clock Input. The clock input signal, nominally 256 kHz, provides the necessary clock for the

Σ-∆

modulator. When this input is static, AD1555 is in the power-down mode.

19

Positive Digital Supply Voltage. 5 V Nominal.

22

AGND3

Analog Ground. Used as the ground reference for the REFIN pin.

23

REFCAP1

DAC Reference Filter. The reference input is internally divided and available at this pin to provide

the reference for the

-

modulator. Connect an external 22

µF (5 V min) tantalum capacitor from

REFCAP1 to AGND3 to filter the external reference noise.

24

REFCAP2

Reference Filter. The reference input is internally divided and available at this pin.

25

REFIN

Reference Input. This input accepts a 3 V level that is internally divided to provide the reference for

the

Σ-∆ modulator.

27

AGND2

Analog Ground.

28

MODIN

Modulator Input. Analog input to the modulator. Normally, this input is directly tied to

PGAOUT output.

AD1556 PIN FUNCTION DESCRIPTIONS

Pin No.

Mnemonic

Description

1, 21, 27, 28,

NC

No Connect

33

2–6

PGA0–PGA4

PGA and MUX Control Inputs. Sets the logic level of CB0-CB4 output pins respectively and the

state of the corresponding bit in the configuration register upon RESET or when in hardware mode.

Refer to Table III.

7–9

BW0–BW2

Output Rate Control Inputs. Sets the digital filter decimation rate and the state of the correspond-

ing bit in the configuration register upon RESET or when in hardware mode. Refer to the Filter

Specifications and Table VI.

10

H/

S

Hardware/Software Mode Select. Determines how the device operation is controlled. In hardware

mode, H/

S is high, the state of hardware pins set the mode of operation. When H/S is low, a write

sequence to the Configuration Register or a previous write sequence sets the device operation.

11, 22, 44

Positive Digital Supply Voltage. 3.3 V or 5 V nominal.

12, 23, 24, 34

DGND

Digital Ground

13

SCLK

Serial Data Clock. Synchronizes data transfer to either write data on the DIN input pin or read

data on the DOUT output pin.