ADR1581
Output turn-on time is modified when an external noise-reduction
filter is used. When present, the time constant of the filter dom-
inates the overall settling.
Attempts to drive a large capacitive load (in excess of 1000 pF) may
result in ringing, as shown in the step response (see Figure 22). This
is due to the additional poles formed by the load capacitance and
the output impedance of the reference. A recommended method
of driving capacitive loads of this magnitude is shown in Figure 19.
A resistor isolates the capacitive load from the output stage,
whereas the capacitor provides a single-pole low-pass filter
and lowers the output noise.
2.4V
V
0V
IN
OUTPUT ERROR
1mV/DIV, 2µs/DIV
2.0V
V
1.8V
IN
OUTPUT
0.5mV/DIV, 2ms/DIV
Figure 20. Turn-On Settling
C
= 0.01µF
L
TRANSIENT RESPONSE
Many ADCs and DACs present transient current loads to the
reference. Poor reference response can degrade the converter’s
performance.
10mV/DIV
50µs/DIV
Figure 22. Transient Response with Capacitive Load
Figure 21 displays both the coarse and fine settling characteristics
of the device to load transients of 50 μA.
PRECISION MICROPOWER LOW DROPOUT
REFERENCE
1mV/DIV
20mV/DIV
The circuit in Figure 23 provides an ideal solution for creating
a stable voltage reference with low standby power consumption,
low input/output dropout capability, and minimum noise output.
The amplifier both buffers and optionally scales up the ADR1581
output voltage. Output voltages as high as 2.1 V can supply 1 mA of
load current. A one-pole filter connected between the ADR1581
and the OP193 input can be used to achieve low output noise. The
nominal quiescent power consumption is 250 ꢀW.
I
= 150µA + 50µA STEP
R
(a)
(b)
3V
I
= 150µA – 50µA STEP
R
28.7kΩ
205Ω
OP193
V
= 1.250V
OUT
OR
1mV/DIV
1µs/DIV
20mV/DIV
4.7µF
V
= 1.250 (1 + R2/R3)
OUT
Figure 21. Transient Settling
Figure 21a shows the settling characteristics of the device for an
increased reverse current of 50 μA. Figure 21b shows the response
when the reverse current is decreased by 50 ꢀA. The transients
settle to 1 mV in about 3 ꢀs.
R2
R3
ADR1581
Figure 23. Micropower Buffered Reference
Rev. 0 | Page 9 of 12
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