LMP7701MF/NOPB National Semiconductor, LMP7701MF/NOPB Datasheet - Page 19

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LMP7701MF/NOPB

Manufacturer Part Number
LMP7701MF/NOPB
Description
IC OP AMP PREC 12V RRIO SOT23-5
Manufacturer
National Semiconductor
Series
LMP®r
Datasheet

Specifications of LMP7701MF/NOPB

Amplifier Type
General Purpose
Number Of Circuits
1
Output Type
Rail-to-Rail
Slew Rate
1.1 V/µs
Gain Bandwidth Product
2.5MHz
Current - Input Bias
0.2pA
Voltage - Input Offset
37µV
Current - Supply
790µA
Current - Output / Channel
86mA
Voltage - Supply, Single/dual (±)
2.7 V ~ 12 V, ±1.35 V ~ 6 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
SOT-23-5, SC-74A, SOT-25
Number Of Channels
1
Voltage Gain Db
130 dB
Common Mode Rejection Ratio (min)
88 dB
Input Voltage Range (max)
12 V
Input Voltage Range (min)
2.7 V
Input Offset Voltage
0.2 mV at 5 V
Output Current (typ)
42 mA
Operating Supply Voltage
3 V, 5 V, 9 V
Supply Current
1 mA at 5 V
Maximum Operating Temperature
+ 125 C
Mounting Style
SMD/SMT
Minimum Operating Temperature
- 40 C
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
-3db Bandwidth
-
Lead Free Status / Rohs Status
 Details
Other names
LMP7701MF
LMP7701MFTR
The schematic shown in Figure 12 is a typical circuit which
can be used for pH measurement. The LM35 is a precision
integrated circuit temperature sensor. This sensor is differen-
tiated from similar products because it has an output voltage
linearly proportional to Celcius measurement, without the
need to convert the temperature to Kelvin. The LM35 is used
to measure the temperature of the solution and feeds this
reading to the Analog to Digital Converter, ADC. This infor-
FIGURE 11. Temperature Dependence of a pH Electrode
FIGURE 12. pH Measurement Circuit
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19
mation is used by the ADC to calculate the temperature
effects on the pH readings. The LM35 needs to have a resis-
tor, R
temperatures below 0°C. R
not expected to go below zero.
The output of pH electrodes is usually large enough that it
does not require much amplification; however, due to the very
high impedance, the output of a pH electrode needs to be
buffered before it can go to an ADC. Since most ADCs are
operated on single supply, the output of the pH electrode also
needs to be level shifted. Amplifier A1 buffers the output of
the pH electrode with a moderate gain of +2, while A2 pro-
vides the level shifting. V
V
The LM4140A is a precision, low noise, voltage reference
used to provide the level shift needed. The ADC used in this
application is the ADC12032 which is a 12-bit, 2 channel con-
verter with multiplexers on the inputs and a serial output. The
12-bit ADC enables users to measure pH with an accuracy of
0.003 of a pH unit. Adequate power supply bypassing and
grounding is extremely important for ADCs. Recommended
bypass capacitors are shown in Figure 12. It is common to
share power supplies between different components in a cir-
cuit. To minimize the effects of power supply ripples caused
by other components, the op amps need to have bypass ca-
pacitors on the supply pins. Using the same value capacitors
as those used with the ADC are ideal. The combination of
these three values of capacitors ensures that AC noise
present on the power supply line is grounded and does not
interfere with the amplifiers' signal.
OUT
= −2V
T
in Figure 12, to –V
pH
+ 1.024V.
OUT
T
is not needed if temperatures are
+
at the output of A2 is given by:
in order to be able to read
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