AD846
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 37. Overload Recovery Test Circuit
Figure 35. Recommended Power Supply Bypassing
THEORY OF OPERATION
Figure 38. Overload Recovery Time Photo
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.
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 MΩ. 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 kΩ feedback resistor, this error is one part in 200,000.
For a transimpedance amplifier with 1 MΩ 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
feedback resistor, RF, is somewhat analogous to the input stage
transconductance of a conventional voltage amplifier; and
therefore, if the value of RF is held constant, the closed-loop
bandwidth also remains virtually constant, independent of
closed-loop voltage gain.
Figure 36. AD846 Simplified Schematic
When operated as a closed-loop amplifier, feedback error cur-
rent, IIN: flows into the inverting input terminal and is conveyed
via current mirrors (transistors Q5, Q6, Q7, and Q8) to the
compensation capacitor, CCOMP. The voltage developed across
C
COMP is buffered by the output stage, consisting of transistors
Q9–Q12.
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REV. C
ADI [ ADI ]
