ADR381 AD [Analog Devices], ADR381 Datasheet - Page 11

• Manufacturer Part Number: ADR381
• Description: Precision Low-Drift 2.048 V/2.500 V SOT-23 Voltage References
• Manufacturer: AD [Analog Devices]

reference is then desirable from the point that an additional
operational amplifier is not required for either reinversion
(current-switching mode) or amplification (voltage-switching
mode) of the DAC output voltage. In general, any positive
voltage reference can be converted into a negative voltage refer-
ence through the use of an operational amplifier and a pair of
matched resistors in an inverting configuration. The disadvan-
tage to this approach is that the largest single source of error in
the circuit is the relative matching of the resistors used.
The circuit in Figure 3 avoids the need for tightly matched
resistors with the use of an active integrator circuit. In this circuit,
the output of the voltage reference provides the input drive for
the integrator. The integrator, to maintain circuit equilibrium,
adjusts its output to establish the proper relationship between
the reference’s V
voltage desired can be chosen by simply substituting for the
appropriate reference IC. A precaution should be noted with
this approach: although rail-to-rail output amplifiers work best
in the application, these operational amplifiers require a finite
amount (mV) of headroom when required to provide any load
current. The choice for the circuit’s negative supply should take
this issue into account.
Precision Current Source
Many times in low power applications, the need arises for a pre-
cision current source that can operate on low supply voltages.
As shown in Figure 4, the ADR380/ADR381 can be configured
as a precision current source. The circuit configuration illustrated
is a floating current source with a grounded load. The reference’s
output voltage is bootstrapped across R
the output current into the load. With this configuration, circuit
precision is maintained for load currents in the range from the
reference’s supply current, typically 90 µA to approximately 5 mA.
REV. A
V
IN
1 F
C1
Figure 3. A Negative Precision Voltage Reference
Uses No Precision Resistors
0.1 F
C2
1
OUT
V
ADR380
IN
GND
3
U1
V
and GND. Thus, any negative output
OUT
2
100k
R3
1k
C3
1 F
R4
SET
A1
(R1 + P1), which sets
1 F
+5V
+V
–5V
C4
–V
U2
OP195
100
R5
–V
REF
–11–
Precision High Current Voltage Source
In some cases, the user may want higher output current delivered
to a load and still achieve better than 0.5% accuracy out of the
ADR380/ADR381. The accuracy for a reference is normally
specified on the data sheet with no load. However, the output
voltage changes with load current.
The circuit in Figure 5 provides high current without compro-
mising the accuracy of the ADR380/ADR381. By op amp action,
V
equilibrium, the op amp also drives the N-Ch MOSFET Q1 into
saturation to maintain the current needed at different loads. R2
is optional to prevent oscillation at Q1. In such an approach, hun-
dreds of milliamps of load current can be achieved and the current
is limited by the thermal limitation of Q1. V
O
V
IN
follows V
Figure 5. ADR380/ADR381 for Precision High
Current Voltage Source
1
V
IN
V
ADR380/
1 F
IN
ADR381
Figure 4. A Precision Current Source
C1
REF
GND
3
U1
V
with very low drop in R1. To maintain circuit
OUT
0.1 F
C2
2
A1
1
+8 –15V
+V
–V
V
ADR380
IN
AD820
GND
U1
3
V
OUT
ADR380/ADR381
C1
0.001 F
100
2
R2
100k
R1
C3
1 F
2N7002
ADJUST
IN
Q1
I
SY
= V
R1
P1
R
O
I
L
OUT
R
+ 300 mV.
L
V
O