Datasheet

AD8278/AD8279
Rev. C | Page 17 of 24
POWER SUPPLIES
The AD8278 and AD8279 operate extremely well over a very
wide range of supply voltages. They can operate on a single
supply as low as 2 V and as high as 36 V, under appropriate
setup conditions.
For best performance, the user should ensure that the internal
op amp is biased correctly. The internal input terminals of the
op amp must have sufficient voltage headroom to operate
properly. Proper operation of the part requires at least 1.5 V
between the positive supply rail and the op amp input terminals.
This relationship is expressed in the following equation:
V5.1−+<
+
S
REF
VV
R2R1
R1
For example, when operating on a +V
S
= 2 V single supply and
V
REF
= 0 V, it can be seen from Figure 50 that the op amp input
terminals are biased at 0 V, allowing more than the required 1.5 V
headroom. However, if V
REF
= 1 V under the same conditions, the
input terminals of the op amp are biased at 0.66 V (G = ½). Now
the op amp does not have the required 1.5 V headroom and
cannot function. Therefore, the user must increase the supply
voltage or decrease V
REF
to restore proper operation.
The AD8278 and AD8279 are typically specified at single and
dual supplies, but they can be used with unbalanced supplies as
well; for example, −V
S
= −5 V, +V
S
= +20 V. The difference between
the two supplies must be kept below 36 V. The positive supply
rail must be at least 2 V above the negative supply.
08308-046
R4
R3
R1
R2
R1
R1 + R2
(V
REF
)
R1
R1 + R2
(V
REF
)
V
REF
Figure 50. Ensure Sufficient Voltage Headroom on the Internal Op Amp
Inputs
Use a stable dc voltage to power the AD8278 and AD8279. Noise
on the supply pins can adversely affect performance. Place a
bypass capacitor of 0.1 µF between each supply pin and ground,
as close as possible to each supply pin. Use a tantalum capacitor
of 10 µF between each supply and ground. It can be farther
away from the supply pins and, typically, it can be shared by
other precision integrated circuits.