54626185833202AN584_0| Application Note

54626185833202AN584_0| Application Note

AN-584 APPLICATION NOTE
Using the AD813x

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THEORY OF OPERATION The AD813x differs from conventional op amps by the external presence of an additional input and output. The additional input, VOGM, controls the output commonmode voltage. The additional output is the analog complement of the single output of a conventional op amp. For its operation, the AD813x makes use of two feedback loops as compared to the single loop of conventional op amps. While this provides significant freedom to create various novel circuits, basic op amp theory can still be used to analyze the operation. One of the feedback loops controls the output commonmode voltage, VOUT,cm. Its input is VOCM (Pin 2) and the output is the common-mode, or average voltage, of the two differential outputs (+OUT and OUT). The gain of this circuit is internally set to unity. When the AD813x is operating in its linear region, this establishes one of the operational constraints: VOUT,cm = VOCM. The second feedback loop controls the differential operation. Similar to an op amp, the gain and gain-
shaping of the transfer function is controllable by adding passive feedback networks. However, only one feedback network is required to "close the loop" and fully constrain the operation. But depending on the function desired, two feedback networks can be used. This is possible as a result of having two outputs that are each inverted with respect to the differential inputs. DEFINITION OF TERMS
CF RF RG +IN OUT

Differential voltage refers to 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 )

(1)

V+OUT and V OUT refer to the voltages at the +OUT and OUT terminals with respect to a common reference.
Common-mode voltage refers to the average of two node voltages. The output common-mode voltage is defined as:

VOUT,cm = (V+OUT + V OUT ) / 2
PIN FUNCTION DESCRIPTIONS Pin No. Mnemonic 1 2 IN VOCM Function

(2)

3 4 5 6 7 8

V+ +OUT OUT V NC +IN

Negative Input Voltage applied to this pin sets the common-mode output voltage with a ratio of 1:1. For example, 1 V dc on VOCM will set the dc bias level on +OUT and OUT to 1 V. Positive Supply Voltage Positive Output. Note: The voltage at DIN is inverted at +OUT. Negative Output. Note: The voltage at +DIN is inverted at OUT. Negative Supply Voltage No Connect Positive Input

+DIN VOCM DIN

AD813x
RG IN RF CF +OUT

RL,dm

VOUT,dm

Figure 1. Circuit Definitions

GENERAL USAGE OF THE AD813x Several assumptions are made here for a first-order analysis, which are the typical assumptions used for the analysis of op amps: The input impedances are arbitrarily large and their loading effect can be ignored. The input bias currents are sufficiently small so they can be neglected. The output impedances are arbitrarily low. The open-loop gain is arbitrarily large, which drives the amplifier to a state where the input differential voltage is effectively zero. Offset voltages are assumed to be zero.

REV. 0

Analog Devices, Inc., 2002