Texas Instruments Single 8 MHz Low Power CMOS, EMI Hardened Operational Amplifier, Evaluation module LMV851EVAL/NOPB LMV851EVAL/NOPB Datenbogen
Produktcode
LMV851EVAL/NOPB
R
2
100 k
:
R
1
1 k
:
ADC
Input
uC
SENSOR
R
S
V
S
+
-
+
-
Typical Applications and EMIRR
6
Typical Applications and EMIRR
EMI hardened op amps can be used in a wide range of applications. Applications with sensors that
produce relatively low signal levels especially benefit from the EMI robustness. These small signals can
easily be deteriorated by an interfering RF signal. As the small signals require a large gain of the op amp
circuit, the detected RF signals get amplified as well and might introduce a significant error in the overall
signal processing path. An EMI hardened op amp minimizes the effect of interference that, once detected,
propagates into the subsequent circuitry. Two examples are given to demonstrate the advantage of EMI
hardened op amps:
produce relatively low signal levels especially benefit from the EMI robustness. These small signals can
easily be deteriorated by an interfering RF signal. As the small signals require a large gain of the op amp
circuit, the detected RF signals get amplified as well and might introduce a significant error in the overall
signal processing path. An EMI hardened op amp minimizes the effect of interference that, once detected,
propagates into the subsequent circuitry. Two examples are given to demonstrate the advantage of EMI
hardened op amps:
•
The first example describes a generic signal path application
•
The second example describes a test in which a cell phone interferes with a pressure sensor
application
application
6.1
Signal Path Application
In
, a typical signal path application is shown with a sensor, an op amp and an ADC that connects
to a microcontroller. The sensor can be a Ph-sensor or a thermocouple for instance, that produces a
signal which needs to be measured. This signal is amplified by the op amp to match the ADC input range.
Accordingly the ADC digitizes the signal such that it can be read by a microcontroller for further
processing.
signal which needs to be measured. This signal is amplified by the op amp to match the ADC input range.
Accordingly the ADC digitizes the signal such that it can be read by a microcontroller for further
processing.
Figure 9. Typical Signal Path Application
Suppose that this application is in an aggressive EM environment and that the interfering signal is mainly
received by the sensor and its wiring. Consequently, an RF signal arrives at the input of the op amp.
Further, assume that the RF signal at the input of the op amp is
received by the sensor and its wiring. Consequently, an RF signal arrives at the input of the op amp.
Further, assume that the RF signal at the input of the op amp is
−
20 dBV
P
at 900 MHz and that the gain of
the op amp configuration is 101x. The ADC used has a 10-bit resolution and a 5V input range. Two
examples will be given to demonstrate the effect an interfering signal can have on the ADC measurement
accuracy and thus on the overall performance.
examples will be given to demonstrate the effect an interfering signal can have on the ADC measurement
accuracy and thus on the overall performance.
•
First, a standard op amp without any special EMI robustness is used. Such an op amp can have an
EMIRR of 50 dB at 900 MHz, for instance. This means that for this situation the input referred offset
voltage shifts about 0.32 mV as a result of the RF signal of
EMIRR of 50 dB at 900 MHz, for instance. This means that for this situation the input referred offset
voltage shifts about 0.32 mV as a result of the RF signal of
−
20 dBV
P
. As a result of the gain of 101x
the output voltage shift equals 32 mV.
•
Second, an LMV851 EMI hardened op amp is placed in the application. The LMV851 has an EMIRR of
about 80 dB at 900 MHz. This results in an input referred offset voltage shift of about 10 µV, which is
equivalent to 1 mV shift at the output.
about 80 dB at 900 MHz. This results in an input referred offset voltage shift of about 10 µV, which is
equivalent to 1 mV shift at the output.
The ADC has a resolution of 10 bit with a 5V range. This means that one bit corresponds to 5/1024 = 4.88
mV. To be able to use the full measurement resolution without incorrect readings the error signal should
not be larger than half a bit or 2.44 mV. The standard op amp has an output shift of 32 mV, which is
equivalent to about 7 counts. The output shift of 1 mV for the LMV851 EMI hardened op amp is equivalent
to 0.2-bit.
mV. To be able to use the full measurement resolution without incorrect readings the error signal should
not be larger than half a bit or 2.44 mV. The standard op amp has an output shift of 32 mV, which is
equivalent to about 7 counts. The output shift of 1 mV for the LMV851 EMI hardened op amp is equivalent
to 0.2-bit.
11
SNOA497B – September 2007 – Revised April 2013
AN-1698 A Specification for EMI Hardened Operational Amplifiers
Copyright © 2007–2013, Texas Instruments Incorporated