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Preliminary Technical Data

AD7933/AD7934

REFERENCE

AD7933/

AD7934

ADC

BUFFER

VREFIN/

VREFOUT

Example 2

VIN MAX = VDD + 0.3

VIN MAX = VREF + VREF/2

If VDD = 3.6 V, then VIN MAX = 3.9 V.

Figure 35. Internal Reference Circuit Block Diagram

Therefore, 3 × VREF/2 = 3.6 V.

Alternatively, an external reference can be applied to the

VREFIN/VREFOUT pin of the AD7933/AD7934. An external

reference input is selected by setting the REF bit in the internal

control register to 0. When using an external reference, the

VREFIN/VREFOUT pin should be decoupled to AGND with a 0.1 µF

capacitor. When operating in differential mode, the external

reference input range is 0.1 V to 3.5V, and in single-ended and

pseudo-differential mode, the external reference input range is

0.1 V to VDD. In all cases, the specified reference is 2.5 V.

VREF MAX = 2.6 V

Therefore, when operating with at VDD = 3 V, the value of VREF

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

VDD = 2.7 V, VREF MAX = 2 V.

These examples show that the maximum reference applied to

the AD7933/AD7934 is directly dependant on the value applied

to VDD.

It is important to ensure that, when choosing the reference

value, the maximum analog input range (VIN MAX) is never

greater than VDD + 0.3 V to comply with the maximum ratings

of the device. In pseudo differential mode, the user must ensure

that VREFIN − VIN− ≤ VDD.

The performance of the part at different reference values is

shown in Figures TBD to TBD. 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

AD7933/AD7934 transfer function and add to the specified

full-scale errors on the part.

lists suitable voltage

references available from ADI that could be used, and F

shows a typical connection diagram for an external reference.

Table 11

Table 11. Examples of Suitable Voltage References

The following two examples calculate the maximum VREF input

that can be used when operating the AD7933/AD7934 in

differential mode with a VDD of 5 V and 3 V, respectively.

igure 36

Figure 36. Typical VREF Connection Program

Example 1

Reference

Output

Voltage

Initial Accuracy

(% max)

Operating

Current (µA)

AD780

2.5/3

0.04

1000

ADR421

2.5

0.04

500

ADR420

2.048

0.05

500

VIN MAX = VDD + 0.3

VIN MAX = VREF + VREF/2

AD780

+VIN

GND

TEMP

O/PSELECT

TRIM

VOUT

VREF

2.5V

VDD

NC = NO CONNECT

10nF

0.1

µF

0.1

µF

0.1

µF

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7933/

AD7934*

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

Therefore, 3 × VREF/2 = 5.3 V.

VREF MAX = 3.5 V

Therefore, 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.

Rev. PrG | Page 23 of 32

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