LMP7721MAEVAL National Semiconductor, LMP7721MAEVAL Datasheet - Page 13

• Manufacturer Part Number: LMP7721MAEVAL
• Description: BOARD EVALUATION FOR LMP7721
• Manufacturer: National Semiconductor
• Series: LMP®, PowerWise®r

Specifications of LMP7721MAEVAL

• Channels Per Ic: 1 - Single
• Amplifier Type: General Purpose
• Output Type: Single-Ended
• Slew Rate: 12.8 V/µs
• Current - Output / Channel: 60mA
• Operating Temperature: -40°C ~ 125°C
• Current - Supply (main Ic): 1.3mA
• Voltage - Supply, Single/dual (±): 1.8 V ~ 5.5 V, ±0.9 V ~ 2.75 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMP7721
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The signal conductor and the guard of the triax should be kept
at the same potential; therefore, the leakage current between
them is practically zero. Since triax has an extra layer of in-
sulation and a second conducting sheath, it offers greater
rejection of interference than coaxial cable/connector.
TRANSIMPEDANCE AMPLIFIER EXAMPLE (INVERTING
CONFIGURATION)
A transimpedance amplifier converts a small amount of cur-
rent into voltage. The transfer function of a transimpedance
amplifier is V
simpedance amplifier.
The current is generated by a photodiode. The amount of the
current is so small that it requires a large gain from the tran-
simpedance amplifier in order to transform the miniscule cur-
rent into easily detectable voltages. The larger the gain, the
larger the value of R
caused by I
1000 MΩ, and an op amp with 3 nA of I
I
matically reduced to 3 µV by using the LMP7721.
Photodiodes are high impedance sensors which require care-
ful design of the associated signal conditioning circuitry in
order to meet the system challenges. CMOS input op amps
are often used in transimpedance applications as they have
extremely high input impedance. A triaxial cable is recom-
mended for its very low noise pick-up.
A MOS input stage with ultra low input bias current, negligible
input current noise, and low input voltage noise allows the
LMP7721 to provide high fidelity amplification. In addition, the
LMP7721 has a 17 MHz gain bandwidth product, which en-
ables high gain at wide bandwidth. A rail-to-rail output swing
at 5.5V power supply allows detection and amplification of a
wide range of input currents. These properties make the
LMP7721 ideal for transimpedance amplification.
Figure 3 is an example of the LMP7721 used as a tran-
simpedance amplifier.
bias
FIGURE 2. Photodiode Transimpedance Amplifier
*R
F
error at the output will be 3V! This error can be dra-
out
bias
= −I
*R
F
in
F
increases. For example, if R
needed. When R
* R
F
. Figure 2 shows a typical tran-
F
is larger, the error
bias
is used, the
20204089
F
is
13
The current generated by the photodiode is fed to the signal
conductor of the triax and then sent to the inverting input of
the LMP7721. The LMP7721’s non-inverting input is biased
at V
non-inverting input is a low impedance node and hence is
used to drive the LMP7715 which acts as a guard driver. The
output of the guard driver is connected to the guard of the triax
via a 100Ω isolation resistor. Ideally, the inverting and the
non-inverting inputs of the amplifier are kept at the same po-
tential through the operation of the amplifier. By connecting
the signal conductor to the inverting input and letting the non-
inverting input drive the guard, the signal conductor and the
guard are kept at the same potential which prevents leakage
from the signal source.
pH ELECTRODE AMPLIFIER EXAMPLE (NON-
INVERTING CONFIGURATION)
The output of a pH electrode ranges from 415 mV to −415 mV
as the pH changes from 0 to 14 at 25°C. The output
impedance of a pH electrode is extremely high, ranging from
10 MΩ to 1000 MΩ. The ultra low input bias current of the
LMP7721 allows the voltage error produced by the input bias
current and electrode resistance to be minimal. For example,
the output impedance of the pH electrode used is 10 MΩ, if
an op amp with 3 nA of I
this amplifier’s input bias current and the source resistance of
the pH electrode is 30 mV! This error can be greatly reduced
to 30 nV by using the LMP7721.
FIGURE 3. LMP7721 as Transimpedance Amplifier
REF
/2 for level shifting purposes. In this application, the
bias
is used, the error caused due to
www.national.com
20204090