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PRELIMINARY TECHNICAL DATA

–16–

AD7450

EXTERNAL

VREF (2.5V)

VREF

VIN+

AD7450

VIN-

2.5V

3.75V

1.25V

2.5V

3.75V

1.25V

Figure 16. Using an RF Transformer to Generate

Differential Inputs

REFERENCE SECTION

An external reference source is required to supply the

reference to the AD7450.

This reference input can range

from 100 mV to 2.5 V.

With a 5V power supply, the

specified and maximum reference is 2.5V.

With a 3V

power supply, the specified reference is 1.25V and the

maximum reference is 2.2V. In both cases, the reference is

functional from 100mV. It is important to note that as the

reference input moves closer to the maximum reference

input, the performance improves.

When operating the

device from VDD = 2.7V to 3.3V, the maximum analog

input range (VINmax) must never be greater than VDD +

0.3V to comply with the maximum ratings of the device.

For example:

VINmax = VDD + 0.3

VINmax = VREF + VREF/2

If VDD = 3.3V

then VINmax = 3.6 V

Therefore 3xVREF/2 = 3.6 V

VREF max = 2.4 V

Therefore, when operating at VDD = 3.3 V, the value of

VREF can range from 100mV to a maximum value of 2.4V.

When VDD = 2.7 V, VREF max = 2 V.

When operating from VDD = 4.75 V to 5.25 V, there is

no need to worry about the maximum analog input

relation to VDD as the maximum VREF is 2.5 V resulting

the maximum analog input span being 3.75 V which is not

close to VDD.

The performance of the part at different reference values is

shown in TPC9 to TPC13 and in TPC16 and TPC17.

The value of the reference sets the analog input span and

the common mode voltage range. Errors in the reference

source will result in gain errors in the AD7450 transfer

function and will add to specified full scale errors on the

part.

A capacitor of 0.1µF should be used to decouple the

VREF pin to GND. Table I lists examples of suitable volt-

age references that could be used that are available from

Analog Devices and Figure 17 shows a typical connection

diagram for the VREF pin.

Table I Examples of Suitable Voltage References

Reference Output

Initial

Operating

Voltage

Accuracy

Current

(% max)

(µA)

AD589

1.235

1.2-2.8

AD1580

1.225

0.08-0.8

REF192

2.5

0.08-0.4

REF43

2.5

0.06-0.1

600

AD780

2.5

0.04-0.2

1000

VREF

AD7450*

VDD

VIN

Temp

GND

Trim

Vout

OpSel

0.1µF

VDD

0.1µF

10nF

*ADDITIONA L PINS OMITTED FOR CLARITY

AD780

Figure 17. Typical VREF Connection Diagram

SINGLE ENDED OPERATION

When supplied with a 5 V power supply, the AD7450 can

handle a single ended input.

The design of this part is

optimized for differential operation so with a single ended

input performance will degrade. Linearity will degrade by

typically 0.2LSBs, Zero Code and the Full Scale Errors

will degrade by typically 2LSBs and AC performance is

not guaranteed.

To operate the AD7450 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

mid-scale 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

voltage span of 2 x VREF to make use of the full dynamic

range of the part. The input signal will therefore have peak

to peak values of Common Mode ±VREF.

If the analog

input is unipolar then an opamp in a non-inverting

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

Because the ADC operates from a single supply, it will be

necessary to level shift ground based bipolar signals to

comply with the input requirements.

An opamp can be

configured to rescale and level shift the ground based bi-

polar signal so it is compatible with the selected input

range of the AD7450 (see Figure 18).

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