Datasheet
AD5379
Rev. B | Page 19 of 28
V
BIAS
FUNCTION
The AD5379 has an on-chip voltage generator that provides a
bias voltage of 4.25 V (minimum). 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 VOUT39. If the
offset and gain features are not used (m and c are left at their
power-on values), the required reference levels can be
calculated as follows:
VREF(+)
min
= (VOUT
max
− VOUT
min
)/3.5
VREF(−)
max
= (AGND + VOUT
min
)/2.5
If the offset and gain features of the AD5379 are used, then the
required output range is slightly different. The chosen output
range 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 required reference levels 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 expected, maximum offset and gain errors.
4. Choose the new required VOUT
max
and VOUT
min
, keeping
the new VOUT limits centered on the nominal values and
assuming REFGND is zero (or equal to AGND). Note that
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 to
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 is too large to calibrate, then adjust the
negative reference value to account for this 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 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 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
V
REF
(−) = −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 V
REF
(+)
min
or below
V
REF
(−)
max
. Modify the gain and offset registers to digitally
downsize the references. In this way, the user can use
almost any convenient reference level, but may reduce
performance by overcompaction of the transfer function.
• Use a combination of these two approaches.