AD846SQ AD [Analog Devices], AD846SQ Datasheet - Page 9

• Manufacturer Part Number: AD846SQ
• Description: 450 V/us, Precision, Current-Feedback Op Amp
• Manufacturer: AD [Analog Devices]

A more detailed examination of the closed-loop transfer func-
tion of the AD846 results in the following equation:
Compare this to the equation for a conventional op amp:
where: C
plifier; g
In the case of the voltage amplifier, the closed-loop bandwidth
decreases directly with increasing values of (1 + R
closed-loop gain. However, for the transimpedance amplifier,
the situation is different. At low gains, where (1 + R
small compared to R
by the internal compensation capacitance of 7 pF and the value
of R
+ R
ventional operational amplifier in which the input stage transcon-
ductance is equal to the inverting terminal input impedance of
the transimpedance amplifier (R
Closed-Loop Gain G(s) =
F
F
/R
, and not by the closed-loop gain. At higher gains, where (1
S
) R
Closed-Loop Gain G(s) =
M
COMP
is the input stage transconductance of the amplifier.
IN
is much larger than R
is the internal compensation capacitor of the am-
F
, the closed-loop bandwidth is controlled



1+ C
IN



COMP
1+
F
, the behavior is that of a con-
= 50 Ω).
C
COMP



g
R
M
F
−R
−R
R
R
+ 1+
S
S



F



F
1+
R
R
R
R
F
S
F
S
F
F

 s


 R

/R
/R



S
S
IN
) R
), the

 s

IN



is
A simple equation can, therefore, be used to determine the band-
width of an amplifier employing the AD846 in the inverting
configuration.
where: The 3 dB bandwidth is in MHz
NOTE: This equation applies only for values of R
10 kΩ and 100 kΩ, and for R
1 kΩ the bandwidth should be estimated from Figure 41.
Figure 41 illustrates the closed-loop voltage gain vs. frequency
of the AD846 for various values of feedback resistor. For com-
parison purposes, the characteristic of a conventional amplifier
having an 80 MHz unity gain bandwidth is also shown.
For the case where R
of –10), the closed-loop bandwidth is approximately 28 MHz. It
should also be noted that the use of a capacitor to shunt R
normal practice for stabilizing conventional op amps, will cause
this amplifier to become unstable because the closed-loop band-
width will increase beyond the stable operating frequency.
A similar approach can be taken to calculate the noise perfor-
mance of the amplifier. A simplified noise model is shown in
Figure 42.
The equivalent mean-square output noise voltage spectral den-
sity will equal:
V
+ 4 kT R
ON
2
= R
G is the closed-loop inverting gain of the AD846
R
F
(
is the feedback resistance in kΩ.
F
F



3 dB Bandwidth =
I
R
R
NN
F
S
+1
)
2
+ 1+
F



= 1 kΩ and R



R
R
LOAD
F
S



2
[V
R
greater than 500 Ω. For R
F
N
S
+ 0.05 1 + G
2
= 100 Ω (closed-loop gain
+ R
(
23
P
(
I
NP
)
2
AD846
)
+ 4 kT R
F
between
F
, a
P
F
]
=