AD7440BRT-R2 datasheet(20/28 Pages) AD

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AD7440/AD7450A

Rev. B | Page 20 of 28

DIGITAL INPUTS

The digital inputs applied to the device are not limited by the

maximum ratings, which limit the analog limits. Instead the

digital inputs applied, CS and SCLK, can go to 7 V and are not

restricted by the VDD + 0.3 V limits as on the analog input.

The main advantage of the inputs not being restricted to the

VDD + 0.3 V limit is that power supply sequencing issues are

avoided. If CS and SCLK are applied before VDD, there is no risk

of latch-up as there would be on the analog inputs if a signal

greater than 0.3 V was applied prior to VDD.

REFERENCE

An external reference source is required to supply the reference

to the device. This reference input can range from 100 mV to

3.5 V. With a 5 V power supply, the specified reference is 2.5 V

and the maximum reference is 3.5 V. With a 3 V power supply,

the specified reference is 2 V and the maximum reference is

2.2 V. In both cases, the reference is functional from 100 mV.

Ensure that, when choosing the reference value for a particular

application, the maximum analog input range (VIN max) is

never greater than VDD + 0.3 V to comply with the maximum

ratings of the device. The following two examples calculate the

maximum VREF input that can be used when operating the

AD7440/AD7450A at a VDD of 5 V and 3 V, respectively.

Example 1

VIN max = VDD + 0.3

VIN max = VREF + VREF/2

If VDD = 5 V, then VIN max = 5.3 V.

Therefore

3 × VREF/2 = 5.3 V

VREF max = 3.5 V

Thus, when operating at VDD = 5 V, the value of VREF can range

from 100 mV to a maximum value of 3.5 V. When VDD = 4.75 V,

VREF max = 3.17 V.

Example 2

VIN max = VDD + 0.3

VIN max = VREF + VREF/2

If VDD = 3 V, then VIN max = 3.3 V.

Therefore,

3 × VREF/2 = 3.3 V

VREF max = 2.2 V

Thus, when operating at VDD = 3 V, the value of VREF can range

from 100 mV to a maximum value of 2.2 V. When VDD = 2.7 V,

VREF max = 2 V.

These examples show that the maximum reference applied to

the AD7440/AD7450A is directly dependent on the value

applied to VDD.

The value of the reference sets the analog input span and the

common-mode voltage range. Errors in the reference source

result in gain errors in the AD7440/AD7450A transfer function

and add to specified full-scale errors on the part. A 0.1 µF

capacitor should be used to decouple the VREF pin to GND.

Figure 38 shows a typical connection diagram for the VREF pin.

Table 6 lists examples of suitable voltage references.

AD780

VIN

GND

TEMP

OPSEL

TRIM

VOUT

AD7440/

AD7450A*

VREF

2.5V

VDD

NC = NO CONNECT

10nF

0.1

µF

0.1

µF

0.1

µF

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 38. Typical VREF Connection Diagram for VDD = 5 V

Table 6. Examples of Suitable Voltage References

Reference

Output

Voltage (V)

Initial

Accuracy (%)

Operating

Current (µA)

AD780

2.5/3

0.04

1000

ADR421

2.5

0.04

500

ADR420

2.048

0.05

500

SINGLE-ENDED OPERATION

When supplied with a 5 V power supply, the AD7440/AD7450A

can handle a single-ended input. The design of these devices is

optimized for differential operation, so with a single-ended

input, performance degrades. Linearity degrades by typically

0.2 LSB, the full-scale errors degrade typically by 1 LSB, and ac

performance is not guaranteed.

To operate the AD7440/AD7450A in single-ended mode, the

VIN+ input is coupled to the signal source, while the VIN– input is

biased to the appropriate voltage corresponding to the midscale

code transition. This voltage is the common mode, which is a

fixed dc voltage (usually the reference). The VIN+ input swings

around this value and should have a voltage span of 2 × VREF to

make use of the full dynamic range of the part. The input signal

therefore has peak-to-peak values of common mode ±VREF. If

the analog input is unipolar, an op amp in a noninverting unity

gain configuration can be used to drive the VIN+ pin. The ADC

operates from a single supply, so it is necessary to level shift

ground-based bipolar signals to comply with the input

requirements. An op amp can be configured to rescale and level

shift the ground-based bipolar signal, so it is compatible with

the selected input range of the AD7440/AD7450A (Figure 39).

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