ADA4530-1ARZ-R7 datasheet(29/52 Pages) AD

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Data Sheet

ADA4530-1

THEORY OF OPERATION

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Rev. C | 29 of 52

GAIN STAGE

The second stage of the amplifier comprises an n-channel metal-

oxide semiconductor (NMOS) differential pair (M3, M4) and folded

cascode transistors (M13 to M20). The amplifier features nested

Miller compensation (C1 to C3).

OUTPUT STAGE

The ADA4530-1 features a complementary common-source output

stage consisting of the M21 and M22 transistors. These transistors

are configured in a Class AB topology and are biased by the

voltage source, V1. This topology allows the output voltage to be

within tens of millivolts of the supply rails, achieving a rail-to-rail

output swing. The output voltage is limited by the output impedance

of the transistors. The output voltage swing is a function of the load

current and can be estimated using the output voltage to the supply

rail vs. load current graphs (see Figure 40 to Figure 45).

GUARD BUFFER

The guard buffer (BUF1) is a unity-gain amplifier that creates a low

impedance replica of the input common-mode voltage. The buffer

input is connected to the noninverting input (IN+). The noninverting

input voltage is approximately equal to the input common-mode

voltage when the main amplifier feedback loop is settled.

The guard buffer uses a three-stage architecture similar to that of

the amplifier. The guard buffer uses a rail-to-rail output stage that

allows the guard voltage to swing within 100 mV of the supply rails.

Because the guard buffer output follows the input common-mode

voltage, this output swing limits the effectiveness of the guard buffer

at low input common-mode voltages. This limit can be seen as a

significant increase in the input bias current at low common-mode

voltages shown in the input bias current vs. common-mode voltage

graphs (see Figure 22 to Figure 33). For this reason, it is not

recommended to operate the circuit with an input common-mode

voltage of less than 100 mV from the V− supply rail.

The guard buffer output voltage can be degraded from excessive

loading. The 1 kΩ output resistance adds 1 µV of guard voltage

error per 1 nA of load current. It is possible to drive the guard offset

voltage out of its specifications with a few tens of nanoamperes

of load current. For this reason, it is not recommended to drive

anything except insulation resistance (see the Insulation Resistance

section and the Guarding section) with the guard buffer. If more

drive strength is needed, the guard voltage can be buffered with a

low offset, low input bias current op amp such as the ADA4661-2.

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