Datasheet

LTC6240/LTC6241/LTC6242
20
624012fe
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 value source and feedback resistors. High frequency
noise from the amplifi er tail current source, I
TAIL
in Fig-
ure 1, couples through the input capacitance and appears
across these large source and feedback resistors. As an
example, the photodiode amplifi er of Figure 15 on the last
page of this data sheet shows the noise results from the
LTC6241 and the results of a competitive CMOS amplifi er.
The LTC6241 output is the ideal noise of a 1M resistor
at room temperature, 130nV√Hz.
Half the Noise
The circuit shown in Figure 3 can be used to achieve even
lower noise voltage. By paralleling 4 amplifi ers the noise
voltage can be lowered by √4, or half as much noise. The
√
–
comes about from an RMS summing of uncorrelated
noise sources. This circuit maintains extremely high input
resistance, and has a 250 output resistance. For lower
output resistance, a buffer amplifi er can be added without
infl uencing the noise.
Stability
The good noise performance of these op amps can be at-
tributed to large input devices in the differential pair. Above
several hundred kilohertz, the input capacitance rises and
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 4), this pole can create excess
phase shift and possibly oscillation. A small capacitor C
F
in parallel with R
F
eliminates this problem.
Low Noise Single-Ended Input to Differential Output
Amplifi er
The circuit on the fi rst page of the data sheet is a low noise
single-ended input to differential output amplifi er, with a
200k input impedance. The very low input bias current
of the LTC6241 allows for these large input and feedback
resistors. The 200k resistors, R1 and R2, along with C1 and
C2 set the –3dB bandwidth to 80kHz. Capacitor C3 is used
to cancel effects of input capacitance, while C4 adds phase
lead to compensate the phase lag of the second amplifi er.
Figure 3. Parallel Amplifi er Lowers Noise by 2x Figure 4. Compensating Input Capacitance
APPLICATIONS INFORMATION
10Ω
6241 F03
–
+
1/4
LTC6242
1k
1k
10Ω
–
+
1/4
LTC6242
1k
1k
10Ω
–
+
1/4
LTC6242
1k
1k
10Ω
–
+
1/4
LTC6242
1k
1k
+2.5
–2.5
V
IN
V
O
+
–
C
IN
C
S
6241 F04
R
F
R
S
OUTPUT
C
F