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Data Sheet

ADA4817-1/ADA4817-2

THEORY OF OPERATION

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Rev. H | 18 of 29

The ADA4817-1/ADA4817-2 are voltage feedback operational am-

plifiers that combine new architecture for FET input operational

amplifiers with the eXFCB process from Analog Devices, resulting

in an outstanding combination of speed and low noise. The innova-

tive high speed FET input stage handles common-mode signals

from the negative supply to within 2.7 V of the positive rail. This

stage is combined with an H-bridge to attain an 870 V/µs slew

rate and low distortion, in addition to 4 nV/√Hz input voltage

noise. The amplifier features a high speed output stage capable

of driving heavy loads sourcing and sinking up to 40 mA of linear

current. Supply current and offset current are laser trimmed for

optimum performance. These specifications make the ADA4817-1/

ADA4817-2 a great choice for high speed instrumentation and high

resolution data acquisition systems. Their low noise, picoampere

input current, precision offset, and high speed make them superb

preamps for fast photo-diode applications.

CLOSED-LOOP FREQUENCY RESPONSE

The ADA4817-1/ADA4817-2 are classic voltage feedback amplifiers

with an open-loop frequency response that can be approximated

as the integrator response shown in Figure 54. Basic closed-loop

frequency response for inverting and noninverting configurations

can be derived from the schematics shown in Figure 52 and Figure

53.

Figure 52. Noninverting Configuration

Figure 53. Inverting Configuration

NONINVERTING CLOSED-LOOP FREQUENCY

RESPONSE

Solving for the transfer function,

VOVI

= 2π×fCROSSOVERRG+RF

RF+RG S+2π×fCROSSOVER×RG

(4)

where:

fCROSSOVER is the frequency where the open-loop gain of the

amplifier equals 0 dB.

VO is the output voltage.

VI is the input voltage.

At dc,

VOVI

=RF+RGRG

(5)

The closed-loop −3 dB frequency is

f−3dB=fCROSSOVER× RGRF+RG

(6)

INVERTING CLOSED-LOOP FREQUENCY

RESPONSE

Solving for the transfer function,

VOVI

= −2π×fCROSSOVER×RF

RF+RG S+2π×fCROSSOVER×RG

(7)

At dc

VOVI

=−RFRG

(8)

Solve for closed-loop −3 dB frequency using Equation 6.

Figure 54. Open-Loop Gain vs. Frequency

The closed-loop bandwidth is inversely proportional to the noise

gain of the op amp circuit, (RF + RG)/RG. This simple model is

accurate for noise gains above 2. The actual bandwidth of circuits

with noise gains at or below 2 is higher than those predicted with

this model due to the influence of other poles in the frequency

response of the real op amp.

Figure 55 shows the dc errors of the voltage feedback amplifier. For

both inverting and noninverting configurations,

VOUT error =Ib+×RS RG+RFRG −Ib−

×RF+VOS RG+RFRG

(9)

where Ib is the bias current.

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