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Data Sheet

AD7381-4

Rev. 0 | Page 15 of 31

ADC TRANSFER FUNCTION

The AD7381-4 uses a 2.5 V to 3.3 V reference. The AD7381-4

converts the differential voltage of the analog inputs (AINx+ and

AINx−) into a digital output.

The conversion result is MSB first, twos complement. The LSB

size is (2 × VREF)/2N, where N is the ADC resolution. The ADC

resolution is determined by the resolution of the device chosen

and if resolution boost mode is enabled. Table 9 outlines the

LSB size expressed in microvolts for different resolutions and

reference voltage options.

The ideal transfer characteristic of the AD7381-4 is shown in

Figure 30.

011...111

ANALOG INPUT

+FSR – 1.5LSB

+FSR – 1LSB

–FSR + 1LSB

–FSR

–FSR + 0.5LSB

011...110

011...101

100...010

100...001

100...000

Figure 30. ADC Ideal Transfer Function (FSR = Full-Scale Range)

Table 9. LSB Size

Resolution

(Bits)

2.5 V Reference

(µV)

3.3 V Reference

(µV)

305.2

402.8

76.3

100.7

VREF

AINx+

AINx+–

VCM

VREF

VCM

0.1µF

1µF

100Ω

AINA+

VLOGIC

SDI

SDOA

SDOB

SCLK

REGCAP

REFCAP

GND

VCC

REFIO

AINC+

AINA+–

AINC–

AIND+

AIND–

DIGITAL HOST

(MICROPROCESSOR/FPGA)

EXPOSED

PAD

AD7381-4

100Ω

VCM = VREF ÷ 2

VREF = 2.5V TO 3.3V

3.0V TO 3.6V

1.65V TO 3.6V

V–

V+

V–

V+

V–

V+ = 5V

V+

10k

Ω

REF

LDO

1µF

10k

Ω

LDO

NOTES

1. V– IS THE EXTERNAL SUPPLY VOLTAGE (–2.5 V) FOR THE DRIVER AMPLIFIER.

2. PLACE DECOUPLING CAPACITORS CLOSE TO (IDEALLY, RIGHT UP AGAINST)

THE DEVICE SUPPLY PINS AND REFERENCE PIN.

Figure 31. Typical Application Circuit

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