ADR1581

Rev. 0 | Page 9 of 12

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

0V

2.4V

OUTPUT ERROR

1mV/DIV, 2µs/DIV

OUTPUT

0.5mV/DIV, 2ms/DIV

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.

1.8V

2.0V

50µs/DIV

10mV/DIV

Figure 20. Turn-On Settling

TRANSIENT RESPONSE

Many ADCs and DACs present transient current loads to the

reference. Poor reference response can degrade the converter’s

performance.

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

1µs/DIV

1mV/DIV

20mV/DIV

(a)

(b)

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 μW.

3V

28.7kΩ

ADR1581

OP193

OR

R3

R2

4.7µF

205Ω

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 μA. The transients

settle to 1 mV in about 3 μs.

Figure 23. Micropower Buffered Reference