D844 datasheet(12/20 Pages) AD

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AD844

Rev. F | Page 12 of 20

UNDERSTANDING THE AD844

The AD844 can be used in ways similar to a conventional op

amp while providing performance advantages in wideband

applications. However, there are important differences in the

internal structure that need to be understood to optimize the

performance of the AD844 op amp.

OPEN-LOOP BEHAVIOR

Figure 28 shows a current feedback amplifier reduced to essen-

tials. Sources of fixed dc errors, such as the inverting node bias

current and the offset voltage, are excluded from this model.

The most important parameter limiting the dc gain is the

transresistance, Rt, which is ideally infinite. A finite value of Rt

is analogous to the finite open-loop voltage gain in a conven-

tional op amp.

The current applied to the inverting input node is replicated by

the current conveyor to flow in Resistor Rt. The voltage developed

across Rt is buffered by the unity gain voltage follower. Voltage

gain is the ratio Rt/RIN. With typical values of Rt = 3 MΩ and

RIN = 50 Ω, the voltage gain is about 60,000. The open-loop

current gain, another measure of gain that is determined by the

beta product of the transistors in the voltage follower stage (see

Figure 31), is typically 40,000.

IIN

RIN

IIN

Figure 28. Equivalent Schematic

The important parameters defining ac behavior are the

transcapacitance, Ct, and the external feedback resistor (not

shown). The time constant formed by these components is

analogous to the dominant pole of a conventional op amp and

thus cannot be reduced below a critical value if the closed-loop

system is to be stable. In practice, Ct is held to as low a value as

possible (typically 4.5 pF) so that the feedback resistor can be

maximized while maintaining a fast response. The finite RIN

also affects the closed-loop response in some applications.

The open-loop ac gain is also best understood in terms of the

transimpedance rather than as an open-loop voltage gain. The

open-loop pole is formed by Rt in parallel with Ct. Because Ct is

typically 4.5 pF, the open-loop corner frequency occurs at about

12 kHz. However, this parameter is of little value in determining

the closed-loop response.

RESPONSE AS AN INVERTING AMPLIFIER

Figure 29 shows the connections for an inverting amplifier.

Unlike a conventional amplifier, the transient response and the

small signal bandwidth are determined primarily by the value of

the external feedback resistor, R1, rather than by the ratio of

R1/R2 as is customarily the case in an op amp application. This

is a direct result of the low impedance at the inverting input. As

with conventional op amps, the closed-loop gain is −R1/R2.

The closed-loop transresistance is the parallel sum of R1 and Rt.

Because R1 is generally in the range of 500 Ω to 2 kΩ and Rt is

about 3 MΩ, the closed-loop transresistance is only 0.02% to

0.07% lower than R1. This small error is often less than the

resistor tolerance.

When R1 is fairly large (above 5 kΩ) but still much less than Rt,

the closed-loop HF response is dominated by the time constant

R1 Ct. Under such conditions, the AD844 is overdamped and

provides only a fraction of its bandwidth potential. Because of

the absence of slew rate limitations under these conditions, the

circuit exhibits a simple single-pole response even under large

signal conditions.

In Figure 29, R3 is used to properly terminate the input if desired.

R3 in parallel with R2 gives the terminated resistance. As R1 is

lowered, the signal bandwidth increases, but the time constant

R1 Ct becomes comparable to higher order poles in the closed-

loop response. Therefore, the closed-loop response becomes

complex, and the pulse response shows overshoot. When R2

is much larger than the input resistance, RIN, at Pin 2, most of

the feedback current in R1 is delivered to this input, but as R2

becomes comparable to RIN, less of the feedback is absorbed at

Pin 2, resulting in a more heavily damped response. Consequently,

for low values of R2, it is possible to lower R1 without causing

instability in the closed-loop response. Table 3 lists combinations

of R1 and R2 and the resulting frequency response for the circuit

of Figure 29. Figure 16 shows the very clean and fast ±10 V

pulse response of the AD844.

VIN

VOUT

OPTIONAL

AD844

Figure 29. Inverting Amplifier

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