AD7450ARM Просмотр технического описания (PDF) - Analog Devices

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AD7450ARM

Differential Input, 1MSPS, 12-Bit ADC in µSO-8 and S0-8

Analog Devices 

AD7450

PRELIMINARY TECHNICAL DATA

3.75V

Table I Examples of Suitable Voltage References

2.5V

1.25V

Reference Output

Initial

Operating

R

R

VIN+

C

AD7450

Voltage

Accuracy

(% max)

Current

(µA)

AD589 1.235

1.2-2.8

50

R

VIN-

VREF

AD1580 1.225

0.08-0.8

50

3.75V

REF192 2.5

0.08-0.4

45

2.5V

REF43

2.5

0.06-0.1

600

1.25V

AD780

2.5

0.04-0.2

1000

EXTERNAL

VREF (2.5V)

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.

AD780

NC 1

OpSel 8 NC

VDD

2 VIN

7 NC

0.1µF 10nF

0.1µF

3 Temp Vout 6

4 GND Trim 5 NC

VDD

AD7450*

VREF

0.1µF

*ADDITIONAL PINS OMITTED FOR CLARITY

Figure 17. Typical VREF Connection Diagram

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

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.

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 in

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.

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

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

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).

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.

–16–

REV. PrJ

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