ꢇ ꢈꢉ ꢠꢡ ꢢ
ꢇ ꢈꢉ ꢠꢡ ꢣ
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SBOS051B − OCTOBER 1995 − REVISED FEBRUARY 2005
Operation at very low supply voltage requires careful
attention to assure that the input voltages remain within
their linear range. Voltage swing requirements of
internal nodes limit the input common-mode range with
low power supply voltage. Typical performance curves,
“Input Common-Mode Range vs Output Voltage” show
the range of linear operation for 15V, 5V, and 2.5V
supplies.
V+
10.0V
6
REF102
2
R1
R2
4
Pt100
Cu
Cu
K
VO
+5V
RG
INA128
Ref
2.5V − ∆V
R3
100Ω = Pt100 at 0°C
VO
RG
INA128
Ω
300
Ref
2.5V + ∆V
SEEBECK
ISA
COEFFICIENT
TYPE
MATERIAL
(µV/5C)
R , R
1
2
E
+ Chromel
− Constantan
+ Iron
− Constantan
+ Chromel
− Alumel
58.5
66.5kΩ
76.8kΩ
97.6kΩ
102kΩ
J
50.2
39.4
38.0
K
T
Figure 4. Bridge Amplifier
+ Copper
− Constantan
−
Figure 6. Thermocouple Amplifier with RTD
Cold-Junction Compensation
VO
VIN
+
RG
INA128
Ref
R1
1M
VIN
C
0.1µF
1
Ω
−
IO
+
@ G
R1
R1
VIN
+
RG
INA128
Ref
IB
1
=
f
−3dB
π
2 R1C1
OPA130
A1
= 1.59Hz
IO
Load
A
I
ERROR
1.5nA
50pA
1pA
1
B
OPA177
OPA131
OPA602
OPA128
Figure 5. AC-Coupled Instrumentation Amplifier
75fA
Figure 7. Differential Voltage to Current Converter
Ω
RG = 5.6k
Ω
2.8k
G = 10
VO
LA
RG/2
INA128
Ref
RA
Ω
2.8k
Ω
VG
390k
VG
1/2
NOTE: Due to the INA128’s current-feedback
OPA2131
1/2
OPA2131
Ω
10k
RL
topology, V is approximately 0.7V less than
G
the common-mode input voltage. This DC offset
in this guard potential is satisfactory for many
guarding applications.
Ω
390k
Figure 8. ECG Amplifier with Right-Leg Drive
11
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