LMP2234AMT/NOPB National Semiconductor, LMP2234AMT/NOPB Datasheet - Page 16

• Manufacturer Part Number: LMP2234AMT/NOPB
• Description: IC AMP PREC QUAD LO PWR 14TSSOP
• Manufacturer: National Semiconductor
• Series: LMP®, PowerWise®r

Specifications of LMP2234AMT/NOPB

• Amplifier Type: General Purpose
• Number Of Circuits: 4
• Output Type: Rail-to-Rail
• Slew Rate: 0.058 V/µs
• Gain Bandwidth Product: 130kHz
• Current - Input Bias: 0.02pA
• Voltage - Input Offset: 10µV
• Current - Supply: 36µA
• Current - Output / Channel: 30mA
• Voltage - Supply, Single/dual (±): 1.6 V ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 14-TSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• -3db Bandwidth: -
• Other names: *LMP2234AMT; *LMP2234AMT/NOPB; LMP2234AMT

www.national.com
Figure 2 shows a schematic of this input voltage noise reduc-
tion circuit using the LMP2234. Typical resistor values are:
R
PRECISION INSTRUMENTATION AMPLIFIER
Measurement of very small signals with an amplifier requires
close attention to the input impedance of the amplifier, the
gain of the signal on the inputs, and the gain on each input of
the amplifier. This is because the difference of the input signal
on the two inputs is of interest and the common signal is con-
sidered noise. A classic circuit implementation that is used is
an instrumentation amplifier. Instrumentation amplifiers have
a finite, accurate, and stable gain. They also have extremely
high input impedances and very low output impedances. Fi-
nally they have an extremely high CMRR so that the amplifier
can only respond to the differential signal. A typical instru-
mentation amplifier is shown in Figure 3.
G
= 10Ω, R
FIGURE 2. Noise Reduction Circuit
F
= 1 kΩ, and R
O
= 1 kΩ.
20203446
16
There are two stages in this amplifier. The last stage, the out-
put stage, is a differential amplifier. In an ideal case the two
amplifiers of the first stage, the input stage, would be config-
ured as buffers to isolate the inputs. However they cannot be
connected as followers because of mismatch in amplifiers.
That is why there is a balancing resistor between the two. The
product of the two stages of gain will give the gain of the in-
strumentation amplifier. Ideally, the CMRR should be infinite.
However the output stage has a small non-zero common
mode gain which results from resistor mismatch.
In the input stage of the circuit, current is the same across all
resistors. This is due to the high input impedance and low
input bias current of the LMP2234.
By Ohm’s Law:
However:
So we have:
Now looking at the output of the instrumentation amplifier:
Substituting from Equation 4:
This shows the gain of the instrumentation amplifier to be:
Typical values for this circuit can be obtained by
setting: a = 12 and K = 4. This results in an overall gain of
−100.
FIGURE 3. Instrumentation Amplifier
V
O1
–V
O2
= (2a+1)(V
−K(2a+1)
1
–V
2
)
20203436
(1)
(2)
(3)
(4)
(5)
(6)