AD5379 datasheet(19/28 Pages) AD

19 / 28 page

AD5379

Rev. 0 | Page 19 of 28

VBIAS FUNCTION

The AD5379 has an on-chip voltage generator that 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 VOUT39. 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 AD5379 are used, then 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 chosen 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 zero (or equal to AGND). Note that

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, then it is possible

to adjust the negative reference value to account for this 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;

1) Gain Error = ±3%;

=> Maximum Positive Gain Error = +3%

=> Output Range incl. Gain Error = 10 + 0.03 (10) = 10.3 V

2) Offset Error = ±100 mV;

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

=> Output Range incl. Gain Error and Offset Error =

10.3 + 0.2 = 10.5 V

3) 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

VREF(−) = −2.25/2.5 = −0.9 V

If the solution yields inconvenient reference levels, the user can

adopt one of three 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 may reduce perform-

ance by overcompaction of the transfer function.

•

Use a combination of these two approaches.

AD5379 Datasheet(PDF) 19 Page - Analog Devices

AD5379 Datasheet(HTML) 19 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL