AD5378ABC datasheet(19/28 Pages) AD

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Preliminary Technical Data

AD5378

Rev. PrA | Page 19 of 28

VBIAS FUNCTION

The AD5378 on-chip voltage generator provides a bias voltage

of 4.25 V (min). The VBIAS pin is provided for bypassing and

overdriving purposes only. It is not intended to be used as a

supply or a reference. If VREF(+) > 4.25 V, VBIAS must be pulled

high externally to an equal or higher potential such as 5 V. The

external voltage source should be capable of driving a 50 µA

(typical) current sink load.

REFERENCE SELECTION

The voltages applied to VREF(+) and VREF(−) determine the

output voltage range and span on VOUT0 to VOUT31. If the

offset and gain features are not used (m and c are left at their

power-on values), the reference levels required can be

calculated as follows:

VREF(+)min = (VOUTmax − VOUTmin)/3.5

VREF(−)max = (AGND + VOUTmin)/2.5

If the offset and gain features of the AD5378 are used, the

output range required is slightly different. The output range

chosen should take into account the offset and gain errors that

need to be trimmed out. Therefore, the output range should be

larger than the actual required range.

The reference levels required can be calculated as follows:

Identify the nominal output range on VOUT.

Identify the maximum offset span and the maximum gain

required on the full output signal range.

Calculate the new maximum output range on VOUT,

including the maximum offset and gain errors expected.

Choose the new VOUTmax and VOUTmin required, keeping

the new VOUT limits centered on the nominal values and

assuming REFGND is 0 V (or equal to AGND). VDD and

VSS must provide sufficient headroom.

Calculate the values of VREF(+) and VREF(−) as follows:

VREF(+)min = (VOUTmax − VOUTmin)/3.5

VREF(−)max = (AGND + VOUTmin)/2.5

In addition, when using reference values other than those

suggested (VREF(+) = 5 V and VREF(−) = −3.5 V), the expected

offset error component changes as follows:

VOFFSET = 0.125 × (VREF(−)A + 0.7 × VREF(+)A)

where:

VREF(−)A is the new negative reference value.

VREF(+)A is the new positive reference value.

If this offset error too large to calibrated out, it is possible to

adjust the negative reference value to account for this by using

the following equation:

VREF(−)NEW = VREF(−)A − VOFFSET/2.625

Reference Selection Example

Nominal Output Range = 10 V; (−2 V to +8 V)

Offset Error = ±100 mV

Gain Error = ±3%

REFGND = AGND = 0 V

Gain Error = ±3%;

=> Maximum Positive Gain Error = +3%

=> Output Range including Gain Error =

10 + 0.03 (10) = 10.3 V

Offset Error = ±100 mV;

=> Maximum Offset Error Span = 2(100) mV = 0.2 V

=> Output Range including Gain Error and Offset Error =

10.3 + 0.2 = 10.5 V

VREF(+) and VREF(−) Calculation:

Actual Output Range = 10.5 V, that is, −2.25 V to +8.25 V

(centered);

=> VREF(+) = (8.25 + 2.25)/3.5 = 3 V and

VREF(−) = −2.25/2.5 = −0.9 V

If the solution yields inconvenient reference levels, the user can

adopt one of these approaches:

•

Use a resistor divider to divide down a convenient, higher

reference level to the required level.

•

Select convenient reference levels above VREF(+)min or below

VREF(−)max. Modify the gain and offset registers to downsize

the references digitally. In this way, the user can use almost

any convenient reference level, but can reduce performance

by overcompaction of the transfer function.

•

Use a combination of these two approaches.

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