AD8561

–8–

INPUT STAGE AND BIAS CURRENTS

The AD8561 uses a PNP differential input stage that enables

the input common-mode range to extend all the way from the

negative supply rail to within 2.2 V of the positive supply rail.

The input common-mode voltage can be found as the average

of the voltage at the two inputs of the device. To ensure the

fastest response time, care should be taken not to allow the

input common-mode voltage to exceed either of these voltages.

The input bias current for the AD8561 is 3

μA. As with any

PNP differential input stage, this bias current will go to zero on

an input that is high and will double on an input that is low.

Care should be taken in choosing resistor values to be con-

nected to the inputs as large resistors could cause significant

voltage drops due to the input bias current.

The input capacitance for the AD8561 is typically 3 pF. This is

measured by inserting a 5 k

Ω source resistance to the input and

measuring the change in propagation delay.

USING HYSTERESIS

Hysteresis can easily be added to a comparator through the

addition of positive feedback. Adding hysteresis to a comparator

offers an advantage in noisy environments where it is not desir-

able for the output to toggle between states when the input

signal is near the switching threshold. Figure 17 shows a

method for configuring the AD8561 with hysteresis.

R1

SIGNAL

COMPARATOR

R2

Figure 17. Configuring the AD8561 with Hysteresis

The input signal is connected directly to the noninverting input

of the comparator. The output is fed back to the inverting input

through R1 and R2. The ratio of R1 to R1 + R2 establishes the

the window, or the average switching voltage. The Q output will

given in Equation 1:

R1

+ R2

+V

REF

=V

REF

1–

R1

R1

+ R2

⎛

⎝⎜

⎞

⎠⎟

(1)

feedback network. This has the effect of increasing the amount

of hysteresis at high frequencies. This can be useful when com-

paring a relatively slow signal in a high frequency noise environ-

1

2

π C

, the hysteresis

Rev. B