lmp7312max National Semiconductor Corporation, lmp7312max Datasheet - Page 18

• Manufacturer Part Number: lmp7312max
• Description: Lmp7312 Precision Spi-programmable Afe With Differential/single-ended Input/output
• Manufacturer: National Semiconductor Corporation

www.national.com
LMP7312 IN 4-20mA CURRENT LOOP APPLICATION
The 4-20mA current loop shown in
method of transmitting sensor information in many industrial
process-monitoring applications. Transmitting sensor infor-
mation via a current loop is particularly useful when the infor-
mation has to be sent to a remote location over long distances
(1000 feet, or more). The loop’s operation is straightforward:
a sensor’s output voltage is first converted to a proportional
current, with 4mA normally representing the sensor’s zero-
level output, and 20mA representing the sensor’s full-scale
output. Then, a receiver at the remote end converts the 4-20-
mA current back into a voltage which in turn can be further
processed by a computer or display module. A typical 4-20mA
current-loop circuit is made up of four individual elements: a
sensor/transducer; a voltage-to-current converter (commonly
referred to as a transmitter and/or signal conditioner); a loop
power supply; and a receiver/monitor. In loop powered appli-
cations, all four elements are connected in a closed, series
circuit, loop configuration
put voltage whose value represents the physical parameter
being measured. The transmitter amplifies and conditions the
sensor’s output, and then converts this voltage to a propor-
tional 4-20mA dc-current that circulates within the closed
series-loop. The loop power-supply generally provides all op-
erating power to the transmitter and receiver, and any other
LAYOUT CONSIDERATIONS
Power supply bypassing
In order to preserve the gain accuracy of the LMP7312, power
supply stability requires particular attention. The LMP7312
guarantees minimum PSRR of 90dB (or 31.62 µV/V). How-
ever, the dynamic range, the gain accuracy and the inherent
low-noise of the amplifier can be compromised by introducing
and amplifying power supply noise. To decouple the
LMP7312 from supply line AC noise, a 0.1 µF ceramic ca-
pacitor should be located on the supply line, close to the
(Figure
8). Sensors provide an out-
FIGURE 8. LMP7312 in 4-20mA Current Loop application
Figure 8
is a common
18
loop components that require a well-regulated dc voltage. In
loop-powered applications, the power supply’s internal ele-
ments also furnish a path for closing the series loop. The
receiver/monitor, normally a subsection of a panel meter or
data acquisition system, converts the 4-20mA current back
into a voltage which can be further processed and/or dis-
played. The high DC performance of the LMP7312 makes this
difference amplifier an ideal choice for use in current loop AFE
receiver. The LMP7312 has a low input offset voltage and low
input offset voltage drift when configured in amplification
mode. In the circuit shown in
plification mode with a gain of 2V/V and differential output in
order to well match the input stage of the ADC141S626 (SAR
ADC with differential input). The shunt resistor is 100ohm in
order to have a max voltage drop of 2V when 20mA flows in
the loop. The first order filter between the LMP7312 and the
ADC141S626 reduces the noise bandwidth and allows han-
dling input signal up to 2kHz. That frequency has been cal-
culated taking in account the roll off of the filter and ensuring
a gain error less than 1LSB of the ADC141S626. In order to
utilize the maximum number of bits of the ADC141S626 in this
configuration, a 4.1V reference voltage is used. With this sys-
tem, the current of the 4-20mA loop is accurately gained to
the full scale of the ADC and then digitized for further pro-
cessing.
LMP7312. Adding a 10 µF tantalum capacitor in parallel with
the 0.1 µF ceramic capacitor will reduce the noise introduced
to the LMP7312 even further by providing an AC path to
ground for most frequency ranges.
APPENDIX
Offset Voltage and Offset Voltage Drift calculation
Listed in the table below are the calculated values for Offset
Voltage and Offset Voltage Drift based on the max specifica-
tions of these parameters for the core op-amp (for all gain
configurations).
Figure 8
the LMP7312 is in am-
30075561