Rainbow Electronics ADC10321 - Manual (Page 14)

Applications Information
(Continued)
The V
REF
+ F and V
REF
− F pins should each be bypassed to
AGND with 10µF tantalum or electrolytic and 0.1µF ceramic
capacitors. The circuit of Figure 6 may be used if it is desired
to obtain precise reference voltages. The LMC6082 in this
circuit was chosen for its low offset voltage, low voltage
rail-to-rail capability and low cost.
Since the current flowing through the sense lines (those lines
associated with V
REF
+ S and V
REF
− S) is essentially zero,
there is negligible voltage drop across any resistance in
series with these sense pins and the voltage at the inverting
input of the op-amp accurately represents the voltage at the
top (or bottom) of the ladder. The op-amp drives the force
input, forcing the voltage at the ends of the ladder to equal
the voltage at the op-amp’s non-inverting input, plus any
offset voltage. For this reason, op-amps with low V
OS
, such
as the LMC6081 and LMC6082, should be used for this
application.
Voltages at the reference sense pins (V
REF
+ S and V
REF
− S)
should be within the range specified in the Operating Ratings
table (2.3V to 4.0V for V
REF
+ and 1.3V to 3.0V for V
REF
−).
Any device used to drive the reference pins should be able to
source sufficient current into the V
REF
+ F pin and sink suffi-
cient current from the V
REF
− F pin when the ladder is at its
minimum value of 850 Ohms.
The reference voltage at the top of the ladder (V
REF
+) may
take on values as low as 1.0V above the voltage at the
bottom of the ladder (V
REF
−) and as high as (V
A
- 1.0V)
Volts. The voltage at the bottom of the ladder (V
REF
−) may
take on values as low as 1.3 Volts and as high as 3.0V.
However, to minimize noise effects and ensure accurate
conversions, the total reference voltage range (V
REF
+ -
V
REF
−) should be a minimum of 2.0V and a maximum of
2.7V.
10089719
FIGURE 5. Better low component count reference biasing
ADC10321
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