Datasheet

AD1580
Rev. F | Page 9 of 12
TRANSIENT RESPONSE
Many ADC and DAC converters present transient current
loads to the reference. Poor reference response can degrade
the converter’s performance.
Figure 22 displays both the coarse and fine settling characteristics
of the device to load transients of ±50 μA.
(a)
(b)
1µs/DIV
1mV/DIV
20mV/DIV
I
R
= 100µA – 50µA STEP
I
R
= 100µA + 50µA STEP
1mV/DIV
20mV/DIV
00700-022
Figure 22. Transient Settling
Figure 22a shows the settling characteristics of the device for
an increased reverse current of 50 μA. Figure 22b shows the
response when the reverse current is decreased by 50 µA.
The transients settle to 1 mV in about 3 µs.
Attempts to drive a large capacitive load (in excess of 1000 pF) may
result in ringing, as shown in the step response (see Figure 23).
This is due to the additional poles formed by the load capacit-
ance and the output impedance of the reference. A recommended
method of driving capacitive loads of this magnitude is shown
in Figure 20. A resistor isolates the capacitive load from the
output stage, while the capacitor provides a single-pole low-pass
filter and lowers the output noise.
1.8V
2.0V
V
IN
C
L
= 0.01µF
50µs/DIV
10mV/DIV
00700-023
Figure 23. Transient Response with Capacitive Load
PRECISION MICROPOWER LOW DROPOUT
REFERENCE
The circuit in Figure 24 provides an ideal solution for making
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 AD1580
output voltage, V
R
. Output voltages as high as 2.1 V can supply
1 mA of load current. A one-pole filter connected between the
AD1580 and the OP193 input can be used to achieve low output
noise. The nominal quiescent power consumption is 200 µW.
3V
34.8kΩ
AD1580
OP193
V
OUT
= +1.225V
OR
V
OUT
= +1.225 (1 + R2/R3)
R3
R2
4.7µF
205Ω
00700-024
Figure 24. Micropower Buffered Reference
USING THE AD1580 WITH 3 V DATA CONVERTERS
The AD1580 low output drift (50 ppm/°C) and compact submi-
niature SOT-23 package make it ideally suited for today’s high
performance converters in space critical applications.
One family of ADCs for which the AD1580 is well suited is the
AD7714-3 and AD7715-3. The AD7714/AD7715 are charge-
balancing ( ∑-∆) ADCs with on-chip digital filtering intended for
the measurement of wide dynamic range, low frequency signals
such as those representing chemical, physical, or biological
processes. Figure 25 shows the AD1580 connected to the
AD7714-3/AD7715-3 for 3 V operation.
AD7714-3 AND AD7715–3
AD1580
3V
34.8kΩ
REFIN(+)
REFIN(–)
HIGH
IMPEDANCE
>1GΩ
R
SW
5kΩ (TYP)
C
REF
(3pF TO 8pF)
SWITCHING
FREQUENCY DEPENDS
ON
f
CLKIN
00700-025
Figure 25. Reference Circuit for the AD7714-3 and AD7715-3