Technical Article

MS-2212

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November 2011 | Page 3 of 5

©2011 Analog Devices, Inc. All rights reserved.

within the specified input voltage range (IVR) from the data

sheet specification table.

To measure supply current when open loop, such as

operation as a comparator, see Figure 4 and Figure 5. Some

low noise, bipolar op amps have diodes between the inputs

to protect the differential input pair, so the maximum

differential voltage is usually stated in the Absolute

Maximum table as ±0.7 V. If there are internal series

resistors, they are usually in the 500 Ω to 2 kΩ range. The

Absolute Maximum table may state that the maximum

differential voltage is ± supply voltage, but this does not

mean that the part operates. A simplified internal schematic

should be consulted. If one is not provided, a quick call to

the manufacturer can resolve this. In these two

configurations, the choice of resistor values is a little more

critical. The resistor values should be low enough to cause

the differential input voltage to be at least 0.5 V to guarantee

that the output is driven hard into the rail but high enough

not to damage the internal diodes. Values were chosen to

limit the input current to less than 1 mA.

Figure 4. Comparator, output low

Figure 5. Comparator, output high

Table 1 lists the maximum supply current specification from

the data sheets, the measured supply current with the op

supply pins (Figure 3), and the supply current with the

output forced low (Figure 4) and forced high (Figure 5).

Classic op amp and comparator

Table 1 shows that the classic LM358 and LM393 are well

behaved, as expected.

Bipolar Rail-to-Rail Op Amps

All the bipolar rail-to-rail output op amps have supply

current greater than the “maximum” op amp supply current

in one or both comparator circuits. There are several ways to

drive the output stage, so some methods will result in a

supply current increase when driving to one rail or the other.

Without being privy a manufacturer’s internal schematics,

one cannot comment on the behavior.

For the OP284, the second stage and output stage simplified

schematic is shown on the data sheet. See Figure 6.

be a function of the values of R4 and R6. These values are

selected to maximize the op amp performance and minimize

low by Q6/R1/Q1, the supply current will be determined by

R1. Again, the values of R1, I1, etc. are chosen for op amp

performance, not comparator performance.