Datasheet

AD5378
Rev. A | Page 19 of 28
V
BIAS
FUNCTION
The AD5378 on-chip voltage generator provides a bias voltage
of 4.25 V (min). The V
BIAS
pin is provided for bypassing and
overdriving purposes only. It is not intended to be used as a
supply or a reference. If V
REF
(+) > 4.25 V, V
BIAS
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 V
REF
(+) and V
REF
(−) 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:
V
REF
(+)
min
= (VOUT
max
− VOUT
min
)/3.5
V
REF
(−)
max
= (AGND + VOUT
min
)/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:
1. Identify the nominal output range on VOUT.
2. Identify the maximum offset span and the maximum gain
required on the full output signal range.
3. Calculate the new maximum output range on VOUT,
including the maximum offset and gain errors expected.
4. Choose the new VOUT
max
and VOUT
min
required, keeping
the new VOUT limits centered on the nominal values and
assuming REFGND is 0 V (or equal to AGND). V
DD
and
V
SS
must provide sufficient headroom.
5. Calculate the values of V
REF
(+) and V
REF
(−) as follows:
V
REF
(+)
min
= (VOUT
max
− VOUT
min
)/3.5
V
REF
(−)
max
= (AGND + VOUT
min
)/2.5
In addition, when using reference values other than those
suggested (V
REF
(+) = 5 V and V
REF
(−) = −3.5 V), the expected
offset error component changes as follows:
V
OFFSET
= 0.125 × (V
REF
(−)
A
+ 0.7 × V
REF
(+)
A
)
where:
V
REF
(−)
A
is the new negative reference value.
V
REF
(+)
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:
V
REF
(−)
NEW
= V
REF
(−)
A
− V
OFFSET
/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 including 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 including Gain Error and Offset Error =
10.3 + 0.2 = 10.5 V
3. V
REF
(+) and V
REF
(−) Calculation:
Actual Output Range = 10.5 V, that is, −2.25 V to +8.25 V
(centered);
=> V
REF
(+) = (8.25 + 2.25)/3.5 = 3 V and
V
REF
(−) = −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 V
REF
(+)
min
or below
V
REF
(−)
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.