Datasheet

LTC6244
16
6244fb
resistance of the particular application. Specifi cally, these
amplifi ers exhibit the noise of a 4k resistor, meaning it is
desirable to keep the source and feedback resistance at or
below this value, i.e., R
S
+ R
G
||R
FB
≤ 4k. Above this total
source impedance, the noise voltage is not dominated by
the amplifi er.
Noise current can be estimated from the expression i
n
=
√2qI
B
, where q = 1.6 • 10
–19
coulombs. Equating √4kTRΔf
and R
S
√2qI
B
Δf shows that for source resistors below
50G the amplifi er noise is dominated by the source
resistance. See the Typical Characteristics curve Noise
Current vs Frequency.
Proprietary design techniques are used to obtain simulta-
neous low 1/f noise and low input capacitance. Low input
capacitance is important when the amplifi er is used with
high source and feedback resistors. High frequency noise
from the amplifi er tail current source, I
TAIL
in Figure 1,
couples through the input capacitance and appears across
these large source and feedback resistors.
Stability
The good noise performance of these op amps can be
attributed to large input devices in the differential pair.
Above several hundred kilohertz, the input capacitance
can cause amplifi er stability problems if left unchecked.
When the feedback around the op amp is resistive (R
F
), a
pole will be created with R
F
, the source resistance, source
capacitance (R
S
, C
S
), and the amplifi er input capacitance.
In low gain confi gurations and with R
F
and R
S
in even
the kilohm range (Figure 3), this pole can create excess
phase shift and possibly oscillation. A small capacitor C
F
in parallel with R
F
eliminates this problem.
Achieving Low Input Bias Current
The DD package is leadless and makes contact to the PCB
beneath the package. Solder fl ux used during the attach-
ment of the part to the PCB can create leakage current
paths and can degrade the input bias current performance
of the part. All inputs are susceptible because the backside
paddle is connected to V
–
internally. As the input voltage
changes or if V
–
changes, a leakage path can be formed
and alter the observed input bias current. For lowest bias
current, use the LTC6244 in the MS8 package.
Photodiode Amplifi ers
Photodiodes can be broken into two categories: large area
photodiodes with their attendant high capacitance (30pF
to 3000pF) and smaller area photodiodes with relatively
low capacitance (10pF or less). For optimal signal-to-noise
performance, a transimpedance amplifi er consisting of an
inverting op amp and a feedback resistor is most commonly
used to convert the photodiode current into voltage. In low
noise amplifi er design, large area photodiode amplifi ers
require more attention to reducing op amp input voltage
noise, while small area photodiode amplifi ers require
more attention to reducing op amp input current noise
and parasitic capacitances.
Figure 3. Compensating Input Capacitance
APPLICATIONS INFORMATION
+
–
C
IN
C
S
6244 F03
R
F
R
S
OUTPUT
C
F