AD830

Data Sheet

Rev. D | Page 12 of 20

A = 1

Figure 26. Topology Diagram

A = 1

1

Figure 27. Closed-Loop Connection

Precise amplification is accomplished through closed-loop

operation of this topology. Voltage feedback is implemented via

for negative feedback, as shown in Figure 27. An input signal is

ended referred to common. It produces a current signal that is

summed at the high impedance node with the output current

error current necessary to develop the output voltage at the high

impedance node. The error current is usually negligible, so the

transconductances are identical, the differential voltage across

the Y inputs equals the negative of the differential voltage across

This simple relation provides the basis to easily analyze any

function possible to synthesize with the AD830, including any

feedback situation.

single-pole response, excluding the output amplifier and

loading effects. It is important to note that the bandwidth and

general dynamic behavior is symmetrical (identical) for the

noninverting and the inverting connections of the AD830. In

addition, the input impedance and CMRR are the same for

either connection. This is very advantageous and unlike in a

voltage or current feedback amplifier where there is a distinct

difference in performance between the inverting and

noninverting gain. The practical importance of this cannot be

overemphasized and is a key feature offered by the AD830

amplifier topology.

INTERFACING THE INPUT

Common-Mode Voltage Range

The common-mode range of the AD830 is defined by the

amplitude of the differential input signal and the supply voltage.

usually applied to a symmetrical differential signal centered

around a particular voltage, as illustrated in Figure 28. This is

the meaning implied here for common-mode voltage. The

internal circuitry establishes the maximum allowable voltage on

the input or feedback pins for a given supply voltage. This

constraint and the differential input voltage sets the common-

mode voltage limit. Figure 29 shows a curve of the common-

mode voltage range versus the differential voltage for three

supply voltage settings.

Figure 28. Common-Mode Definition

15

0

0

2.0

0.4

0.8

1.2

1.6

6

3

12

9

Figure 29. Input Common-Mode Voltage Range vs. Differential Input Voltage

Differential Voltage Range

The maximum applied differential voltage is limited by the

clipping range of the input stages. This is nominally set at a

2.4 V magnitude and depicted in the cross plot (X-Y) in Figure 30.

The useful linear range of the input stages is set at 2 V but is

actually a function of the distortion required for a particular

application. The distortion increases for larger differential input

voltages. A plot of relative distortion versus the input differential

voltage is shown in Figure 13 and Figure 16. The distortion

characteristics impose a secondary limit to the differential input

voltage for high accuracy applications.