REV. C

AD846

–8–

POWER SUPPLY CONSIDERATIONS

The power supply connections to the AD846 must maintain a

low impedance to ground over a bandwidth of 40 MHz or more.

This is especially important when driving a significant resistive

or capacitive load, since all current delivered to the load comes

from the power supplies. Multiple high quality bypass capacitors

are recommended for each power supply line in any critical

application. A 0.1

µF ceramic and a 2.2 µF electrolytic capacitor

as shown in Figure 35 placed as close as possible to the am-

plifier (with short lead lengths to power supply common) will

assure adequate high frequency bypassing, in most applications.

A minimum bypass capacitance of 0.1

µF should be used for any

application.

Figure 35. Recommended Power Supply Bypassing

THEORY OF OPERATION

The AD846 differs from conventional operational amplifiers

in that it is a transimpedance device rather than a conventional

voltage amplifier. Figure 36 is a simplified schematic of the

AD846. The input stage consists of a pair of transistors, Q1 and

Q2, which are biased by two diode-connected transistors, Q3

and Q4. Transistors Q1 and Q2 have their emitters connected

together, and this common point functions as the inverting in-

put of the amplifier. Correspondingly, the common connection

of the two biasing diodes acts as the noninverting input.

Figure 36. AD846 Simplified Schematic

When operated as a closed-loop amplifier, feedback error cur-

via current mirrors (transistors Q5, Q6, Q7, and Q8) to the

Q9–Q12.

Figure 37. Overload Recovery Test Circuit

Figure 38. Overload Recovery Time Photo

Because the input error signal developed is in the form of a

current, not a voltage, the AD846 differs from conventional

operational amplifiers. This also means that, unlike most opera-

tional amplifiers which rely on negative feedback to produce a

“virtual ground” at the inverting input terminal, this terminal

explicitly has a low impedance.

A unique circuit approach allows the AD846 to realize an open-

loop transimpedance of close to 200 M

Ω. This is nearly three

orders of magnitude greater than that of any other operational

transimpedance amplifier and results in extremely high levels of

dc precision.

As an example, the output voltage gain error is approximately

equal to the value of the feedback resistor divided by the value

of the open-loop transimpedance of the amplifier. That is, when

using a 1 k

Ω feedback resistor, this error is one part in 200,000.

For a transimpedance amplifier with 1 M

Ω transimpedance, this

error is only one part in 1000; such an amplifier would barely be

able to achieve 10-bit precision.

Figure 39 is a simplified three-terminal model for the AD846.

Figure 40 is a simplified three-terminal model for a conventional

voltage op amp. The action of current feedback serves to modify

the behavior of the amplifier under closed-loop conditions. The

transconductance of a conventional voltage amplifier; and

bandwidth also remains virtually constant, independent of

closed-loop voltage gain.