10
LT1169
U
S
A
O
PP
L
IC
AT
I
WU
U
I FOR ATIO
Input offset current = 3pA
Input resistance = 10
13
Ω
Input noise = 3.4µV
P-P
High Speed Operation
The low noise performance of the LT1169 was achieved by
enlarging the input JFET differential pair to maximize the
first stage gain. Enlarging the JFET geometry also increases
the parasitic gate capacitance, which if left unchecked, can
result in increased overshoot and ringing. When the feed-
back around the op amp is resistive (R
F
), a pole will be
created with R
F
, the source resistance and capacitance
(R
S
,C
S
), and the amplifier input capacitance (C
IN
= 1.5pF).
In closed-loop gain configurations with R
S
and R
F
in the
MΩ range (Figure 6), this pole can create excess phase shift
and even oscillation. A small capacitor (C
F
) in parallel with
R
F
eliminates this problem. With R
S
(C
S
+ C
IN
) = R
F
C
F
, the
effect of the feedback pole is completely removed.
Light Balance Detection Circuit
Unity-Gain Buffer with Extended Load Capacitance
Drive Capability
TYPICAL APPLICATIONS N
U
Assume CMRR
A
= 50µV/V or 86dB,
and CMRR
B
= 39µV/V or 88dB,
then ∆CMRR = 11µV/V or 99dB;
if CMRR
B
= –39µV/V which is still 88dB,
then ∆CMRR = 89µV/V or 81dB
By specifying and guaranteeing all of these matching
parameters, the LT1169 can significantly improve the
performance of matching-dependent circuits.
Typical performance of the instrumentation amplifier:
Input offset voltage = 0.8mV
Input bias current = 4pA
Figure 5. Three Op Amp Instrumentation Amplifier
–
+
LT1169 • F06
OUTPUT
R
F
R
S
C
S
C
IN
C
F
Figure 6
4
2
3
OUTPUT
C1
30pF
R7
10k
R6
10k
LT1169 • F05
GAIN =
BANDWIDTH =
INPUT REFERRED NOISE =
WIDEBAND NOISE DC TO 330kHz =
C
L
≤
100
330kHz
8.7nV/√Hz AT 1kHz
5.3µV
RMS
0.01µF
IN–
15V
8
1
–15V
–
+
–
+
1/2
LT1169
IC1
1/2
LT1169
IC1
IN
+
R3
1k
R2
200Ω
R1
1k
7
6
5
R4
1k
1
2
C
L
3
–
+
1/2
LT1169
IC2
R5
1k
LT1169 • TA03
V
OUT
–
+
1/2 LT1169
C1
V
IN
R2
1k
C
L
R1
33Ω
C1 = C
L
≤ 0.1µF
OUTPUT SHORT CIRCUIT CURRENT (∼ 30mA) WILL LIMIT THE RATE
AT WHICH THE VOLTAGE CAN CHANGE ACROSS LARGE CAPACITORS
(I = C
dV
)
dt
LT1169 • TA04
V
OUT
= 1M × (I
1
– I
2
)
PD
1,
PD
2
= HAMAMATSU S1336-5BK
WHEN EQUAL LIGHT ENTERS PHOTODIODES, V
OUT
< 3mV.
–
+
1/2 LT1169
R1
1M
C1
3pF TO 5pF
PD
1
PD
2
I
1
I
2
V
OUT
