LT1168 Linear Technology, LT1168 Datasheet - Page 13

• Manufacturer Part Number: LT1168
• Description: Low Power/ Single Resistor Gain Programmable/ Micropower Precision Instrumentation Amplifier
• Manufacturer: Linear Technology

APPLICATIONS
To avoid any possibility of inadvertently affecting the
signal to be processed, set the common mode time
constant an order of magnitude (or more) larger than the
differential mode time constant. Set the common mode
time constants such that they do not degrade the LT1168
inherent AC CMR. Then the differential mode time con-
stant can be set for the bandwidth required for the appli-
cation. Setting the differential mode time constant close to
the sensor’s BW also minimizes any noise pickup along
the leads. To avoid any possibility of common mode to
differential mode signal conversion, match the common
mode time constants to 1% or better. If the sensor is an
RTD or a resistive strain gauge, then the series resistors
R
instrumentation amplifier.
Figure 5. Adding a Simple RC Filter at the Inputs to an
Instrumentation Amplifier is Effective in Reducing Rectification
of High Frequency Out-of-Band Signals
IN
IN
S1, 2
+
–
can be omitted, if the sensor is in proximity to the
PATIENT
1.6k
1.6k
R
R
S1
S2
GND
–IN
+IN
0.001 F
EXTERNAL RFI
0.001 F
C
C
0.1 F
XCM2
XCM1
FILTER
C
XD
PATIENT/CIRCUIT
PROTECTION/ISOLATION
U
1
0.01 F
C1
INFORMATION
U
1M
R2
LT1112
R
1/2
G
12k
R1
–
+
+
–
3
2
f
– 3dB
LT1168
W
V
V
+
–
500Hz
R3
30k
R4
30k
A
POLE AT 1kHz
V
Figure 6. Nerve Impulse Amplifier
= 101
R
6k
G
U
3
1
2
8
+
–
1168 F05
LT1168
V
G = 10
–3V
OUT
3V
4
7
Nerve Impulse Amplifier
The LT1168’s low current noise makes it ideal for EMG
monitors that have high source impedances. Demonstrat-
ing the LT1168’s ability to amplify low level signals, the
circuit in Figure 6 takes advantage of the amplifier’s high
gain and low noise operation. This circuit amplifies the low
level nerve impulse signals received from a patient at
Pins 2 and 3. R
set a gain of ten. The potential on LT1112’s Pin 1 creates
a ground for the common mode signal. C1 was chosen to
maintain the stability of the patient ground. The LT1168’s
high CMRR ensures that the desired differential signal is
amplified and unwanted common mode signals are at-
tenuated. Since the DC portion of the signal is not impor-
tant, R6 and C2 make up a 0.3Hz highpass filter. The AC
signal at LT1112’s Pin 5 is amplified by a gain of 101 set
by R7/R8 +1. The parallel combination of C3 and R7 form
a lowpass filter that decreases this gain at frequencies
above 1kHz. The ability to operate at 3V on 350 A of
supply current makes the LT1168 ideal for battery-pow-
ered applications. Total supply current for this application
is 1.05mA. Proper safeguards, such as isolation, must be
added to this circuit to protect the patient from possible
harm.
5
6
0.47 F
C2
G
and the parallel combination of R3 and R4
0.3Hz
HIGHPASS
R6
1M
100
R8
5
6
+
–
LT1112
1/2
–3V
15nF
3V
C3
8
4
10k
R7
7
OUTPUT
1V/mV
1168 F06
LT1168
13