ADA4610-1/ADA4610-2/ADA4610-4

Data Sheet

Rev. I | Page 22 of 27

Determine the RF value by the following ratio:

V/ID

where:

V is the desired output voltage of the op amp.

ID is the diode current.

For example, if ID is 100 μA and a 10 V output voltage is

needed, RF must be 100 kΩ. The resistance of the photodiode

(RD) is a junction resistance (see Figure 59).

A typical value for RD is 1000 MΩ. Because RD >> RF, the circuit

behavior is not impacted by the effect of the junction resistance.

The maximum signal bandwidth (fMAX) is

T

F

MAX

C

R

ft

f





2

where ft is the unity-gain frequency of the op amp.

Calculate CF by

ft

R

C

C

F

T

F





2

where ft is the unity-gain frequency of the op amp, and achieves a

phase margin, φM, of approximately 45°.

Increase the CF value to obtain a higher phase margin. Setting

CF to twice the previous value yields approximately φM = 65° and a

maximal flat frequency response, but it reduces the maximum

signal bandwidth by 50%.

Using the previous parameters with a CF ≈ 7 pF, the signal

bandwidth is approximately 250 kHz.

COMPARATOR OPERATION

Although op amps are quite different from comparators,

occasionally an unused section of a dual or a quad op amp can

be used as a comparator; however, this is not recommended for

rail-to-rail output op amps. For rail-to-rail output op amps, the

output stage is generally a ratioed current mirror with bipolar or

MOSFET transistors. With the device operating in open-loop

mode, the second stage increases the current drive to the ratioed

mirror to close the loop. However, the second stage cannot close

the loop, which results in an increase in supply current. With

the ADA4610-1/ADA4610-2/ADA4610-4 op amps configured

as comparators, the supply current can be significantly higher

(see Figure 60 for the supply current vs. the supply voltage for the

ADA4610-4). Configuring an unused section as a voltage follower

with the noninverting input connected to a voltage within the

input voltage range is recommended. The ADA4610-1/ADA4610-2/

ADA4610-4 have a unique output stage design that reduces the

excess supply current but does not entirely eliminate this effect

when the op amp is operating in open-loop mode.

0

1

2

3

4

5

6

7

8

9

0

5

10

15

20

25

30

35

40

VSY (V)

COMPARATOR, VOUT = HIGH

COMPARATOR, VOUT = LOW

FOLLOWER

Figure 60. Supply Current (ISY) vs. Supply Voltage (VSY) for the ADA4610-4 Only