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Texas Instruments Maximize Weigh Scale Accuracy With EMI-Hardened Amplifiers Application notes
Maximize weigh scale accuracy with emi-hardened
amplifiers
Soufiane Bendaoud
Precision Amplifiers
The accuracy of weigh scales is affected by several
factors, including input offset voltage drift, vibration
RFI, and EMI. EMI sources can emanate from light,
long wires, relays, cell phones, and other electronic
equipment in the vicinity. The accuracy of a weigh
scale is affected by radiated and conducted
undesirable signals as these signals can cause
erroneous readings, thereby impacting the sensitivity
of the apparatus.
To avoid such problems, precautionary measures must
be taken. These include shielding, proper grounding,
and filtering. Passive filters at the input and output of
the amplifier are not a trivial task. A simple low-pass
RC filter, whether at the input or output, is likely to
affect the dynamic performance of the amplifier. The
most effective way to reject RF and EMI signals is to
select op amps with integrated filters.
Advantages of EMI-Hardened Op Amps
Texas Instruments precision amplifiers are designed
with integrated filters that are closely matched on
silicon. The additional filters reduce errors through the
signal path feeding into the analog to digital converter.
EMIRR plots are provided in the product data sheet
and, much like PSRR or CMRR, these graphs show
the rejection over a frequency band.
Suppose a non-EMI hardened op amp inherently
provides 50 dB of rejection, is set up in a gain of 100,
and interfaces with a 16-bit analog to digital converter
with a full-scale voltage range of 5 V.
Next, assume we have an RF signal of –20 dBV (0.1
V) at the input of the amplifier. A quick computation
yields 0.31 mV at the input or 0.1V/10^(50/20).
Multiplying by a gain of 101 gives us 32 mV. With a 5V full-scale voltage range and a 16-bit ADC we’ll have
5 / (216) = 76 µV as one LSB.
Taking the initial 32 mV and dividing by 76 µV yields
approximately 420, which represents the loss of digital
counts. Selecting an amplifier like the zero-drift
OPA187 provides 100 dB of EMIRR at 1 GHz. Let us
determine how much improvement we can get by
using the OPA187:
First, we’ll compute the shift at the output as follows:
0.1V/10^5 *101 , which gives us 0.1 mV at the output
of the amplifier. To find the loss of counts we simply
take 0.1 mV and divide by 76 µV, which represents 1
LSB for 16 bits with a full-scale voltage range of 5 V.
We express the equation as: (0.1E-3 / (5 / 65536)),
which yields 1.3 counts! An extraordinary improvement
without compromise.
Check out this clip in TI's video library for some
additional interesting information on How to avoid
electromagnetic interference (EMI).
To better understand how EMI hardened amplifiers,
reduce errors let us consider an example:
Weigh Scale
Display
Zero Drift
Amplifier
High Resolution
ADC
Key PAD
Micro-Controller
RS-232
Line Driver
RS-232 Port
HART
Driver
Load Cell
HART
Figure 1. Typical Block of a Precision Weigh Scale
SBOA334 – January 2019
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Maximize weigh scale accuracy with emi-hardened amplifiers
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180
160
EMIRR IN+ (dB)
140
120
100
80
60
40
20
0
10M
100M
1000M
Frequency (Hz)
C004
Figure 2. EMIRR IN+ vs Frequency
Table 1. Alternative Device Recommendations
2
Device
Unity Gain Bandwidth
Description
OPA189
14 MHz
14MHz, MUX-Friendly, Low-noise, Zero-Drift, RRO, CMOS Precision
Operational Amplifier
OPA188
2 MHz
Precision, Low-Noise, Rail-to-Rail Output, 36V Zero-Drift Operational
Amplifier
OPA388
10 MHz
10MHz, CMOS, Zero-Drift, Zero-Crossover, True RRIO Precision
Operational Amplifier
Maximize weigh scale accuracy with emi-hardened amplifiers
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