AD844

REV. C

–10–

Driving Large Capacitive Loads

Capacitive drive capability is 100 pF without an external net-

work. With the addition of the network shown in Figure 31, the

capacitive drive can be extended to over 10,000 pF, limited by

internal power dissipation. With capacitive loads, the output

speed becomes a function of the overdriven output current

limit. Since this is roughly

±100 mA, under these conditions,

the maximum slew rate into a 1000 pF load is

±100 V/µs. Fig-

ure 32 shows the transient response of an inverting amplifier

(R1 = R2 = 1 k

Ω) using the feed forward network shown in

Figure 31, driving a load of 1000 pF.

Figure 31. Feed Forward Network for Large Capacitive

Loads

of Figure 31

Settling Time

Settling time is measured with the circuit of Figure 33. This cir-

cuit employs a false summing node, clamped by the two

Schottky diodes, to create the error signal and limit the input

signal to the oscilloscope. For measuring settling time, the ratio

of R6/R5 is equal to R1/R2. For unity gain, R6 = R5 = 1 k

Ω,

Ω. For the gain of –10, R5 = 50 Ω, R6 = 500 Ω

put with approximately 275

Ω. Using this network in a unity-

gain configuration, settling time is 100 ns to 0.1% for a –5 V to

Figure 33. Settling Time Test Fixture

DC Error Calculation

Figure 34 shows a model of the dc error and noise sources for

any offset voltage) will appear at the inverting input. The total

V

R1

R2









+ I

serting a resistor in series with the noninverting input will not

necessarily reduce dc error and may actually increase it.

Figure 34. Offset Voltage and Noise Model for the AD844

Noise

Noise sources can be modeled in a manner similar to the dc bias

currents, but the noise sources are Inn, Inp, Vn, and the

V

((Inp R

R1

R2









2

+ (Inn R1)

2

Overall noise can be reduced by keeping all resistor values to a

2 nV/

√Hz, Inp = 10 pA/√Hz, Inn = 12 pA/√Hz, V

to 12 nV/

√Hz. The current noise is dominant in this case, as it

will be in most low gain applications.