AD8132ARZ-R71 datasheet(22/33 Pages) AD

22 / 33 page

AD8132

Rev. I | Page 21 of 32

OPERATIONAL DESCRIPTION

DEFINITION OF TERMS

Differential Voltage

It is the difference between two node voltages. For example, the

output differential voltage (or equivalently output differential

mode voltage) is defined as

VOUT, dm = (V+OUT − V−OUT)

where V+OUT and V−OUT refer to the voltages at the +OUT and

−OUT terminals with respect to a common reference.

Common-Mode Voltage

It is the average of two node voltages. The output common-

mode voltage is defined as

VOUT, cm = (V+OUT + V−OUT)/2

AD8132

+IN

–IN

+DIN

VOCM

–DIN

RL, dm

+OUT

VOUT, dm

–OUT

Figure 64. Circuit Definitions

BASIC CIRCUIT OPERATION

One of the more useful and easy to understand ways to use the

AD8132 is to provide two equal ratio feedback networks. To

match the effect of parasitics, comprise these networks of two

equal value feedback resistors (RF) and two equal value gain

resistors (RG). This circuit is shown in Figure 64.

Like a conventional op amp, the AD8132 has two differential

inputs that can be driven with both differential mode input

voltage (VIN, dm) and common-mode input voltage (VIN, cm).

There is another input to consider (VOCM) on the AD8132 that

is not present on conventional op amps. VOCM is completely

separate from the previous inputs.

There are two complementary outputs whose response can be

defined by a differential mode output (VOUT, dm) and a common-

mode output (VOUT, cm).

Table 10 shows the gain from any type of input to either type

of output.

Table 10. Differential and Common-Mode Gains

Input

VOUT, dm

VOUT, cm

VIN, dm

RF/RG

0 (by design)

VIN, cm

0 (by design)

VOCM

1 (by design)

As listed in Table 10, the differential output (VOUT, dm) is equal to

the differential input voltage (VIN, dm) times RF/RG. In this case, it

does not matter if both differential inputs are driven, or only one

output is driven and the other is tied to a reference voltage, such

as ground. As seen from the two zero entries in the VOUT,dm column,

neither of the common-mode inputs has any effect on this gain.

The gain from VIN, dm to VOUT, cm is 0, and first-order, does not

depend on the ratio matching of the feedback networks. The

common-mode feedback loop within the AD8132 provides a

corrective action to keep this gain term minimized. The term

balance error describes the degree that this gain term differs

from 0.

The gain from VIN,cm to VOUT,dm directly depends on the matching

of the feedback networks. The analogous term for this transfer

function (used in conventional op amps) is common-mode

rejection ratio (CMRR). Therefore, if it has a high CMRR, the

feedback ratios must be well matched.

The gain from VIN, cm to VOUT, cm is ideally 0 and is first-order

independent of the feedback ratio matching. As in the case of

VIN, dm to VOUT, cm, the common-mode feedback loop keeps this

term minimized.

The gain from VOCM to VOUT,dm is ideally 0 when the feedback ratios

are matched only. The amount of differential output signal that is

created by varying VOCM is related to the degree of mismatch in the

feedback networks.

VOCM controls the output common-mode voltage VOUT, cm with a

unity-gain transfer function. With equal ratio feedback networks

(as previously assumed), its effect on each output is the same,

that is the gain from VOCM to VOUT, dm is 0. If not driven, the

output common-mode voltage is set with an internal voltage

divider to a level that is nominally midsupply. It is recommended

that a 0.1 μF bypass capacitor be connected to VOCM.

When unequal feedback ratios are used, the two gains associated

with VOUT, dm become nonzero. This significantly complicates the

mathematical analysis along with any intuitive understanding of

how the part operates.

AD8132ARZ-R71 Folha de dados(PDF) 22 Page - Analog Devices

AD8132ARZ-R71 Folha de dados(HTML) 22 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Folha de dados Download

Ligação URL