Texas Instruments | EMI-Hardened Op Amps Reduce Errors In Pulse Oximeters (Rev. A) | Application notes | Texas Instruments EMI-Hardened Op Amps Reduce Errors In Pulse Oximeters (Rev. A) Application notes

Texas Instruments EMI-Hardened Op Amps Reduce Errors In Pulse Oximeters (Rev. A) Application notes
EMI-Hardened Op Amps Reduce Errors In Pulse
Oximeters
Soufiane Bendaoud
Medical devices such as infusion pumps, EKGs and
pulse oximeters are designed with great care for
electromagnetic compatibility (EMC). In other words,
they must be compatible with their environment, and
not cause any interference.
316 mV is present at the output of the op amp [ (100
mV / 31.6 ) × 100 ]. Now assume the output is fed to a
12-bit ADC with a 5-V FSR.
Before the creation of electromagnetic interference
(EMI)-hardened amplifiers, board and system-level
engineers struggled to come up with the best possible
filtering schemes without compromising signal chain
performance, especially that of the analog front end.
Operational amplifiers needed special care and
attention due to common mishaps, such as placing a
capacitor across the inputs of the amplifier.
5 V / (212) = 1.22 mV. Divide the output of the op amp
by 1.22 mV (316 mV / 1.22 mV) to determine a loss of
nearly 260 counts.
While the goal was to eliminate undesirable voltage
spikes at the output of the amplifier, it usually resulted
in unstable circuits.
The loss of counts caused the injected signal (EMI)
can be computed as follows:
Using an op amp like the OPA192 reduces the count
loss to roughly eight.
When making a design more immune to EMI without
compromising the circuit performance, look up TI’s
precision amplifiers portfolio.
See this clip for additional information on avoiding
electromagnetic interference (EMI).
Similarly, filters on the output, passive, low-pass filters
(RC) particularly, can be a headache. Lowering the
cutoff frequency often implies using a large capacitor,
which not every op amp can drive. Conversely,
increasing the resistor value causes gain errors.
Red Light
IR Light
DAC
Gain/Filter
Put The Op Amp To Work
TIA
The easiest way to deal with rejecting unwanted RF
signals and electromagnetic interference is selecting
active components with integrated filters. Every TI op
amp designed in the last seven years provides such
filters.
ADC
Voltage
Reference
To better appreciate the benefits of EMI filters in the
amplifier, consider an example:
A first-order filter at 10 MHz rejects 40 dB at 1 GHz.
However, a 10 MHz device with a cut-off at 100MHz
has a rejection of only 20 dB at 1 GHz.
EMI errors can have serious consequences on the
system. Suppose 100 mV is injected into an amplifier
with gain of 100. This example uses an op amp with
no EMI filters, but still provides 30 dB of rejection (1
GHz).
Figure 1. Typical Block Diagram for EKG
160.0
140.0
PRF = -10 dBm
VSUPPLY = ±18 V
VCM = 0 V
120.0
EMIRR IN+ (dB)
The choice of amplifiers ranges from the INA828 highvoltage instrumentation amplifier to the ubiquitous
OPA192, which comes in single, dual, and quad
options.
Power Management
TIA
To determine whether the EMI rejection is sufficient for
the application, a plot like the CMRR and PSRR,
called the EMIRR (EMI rejection ratio), is provided in
the datasheet.
SNOAA31A – April 2019 – Revised August 2019
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DAC
100.0
80.0
60.0
40.0
20.0
0.0
10M
100M
1G
Frequency (Hz)
10G
C017
Figure 2. OPA192 EMI Rejection Ratio Plot Versus
Frequency
EMI-Hardened Op Amps Reduce Errors In Pulse Oximeters Soufiane Bendaoud
Copyright © 2019, Texas Instruments Incorporated
1
www.ti.com
Table 1. Alternative Device Recommendations
DEVICE
UNITY GAIN
BANDWIDTH
DESCRIPTION
OPA320
20 MHz
Precision, Zero-Crossover, 20 MHz, 0.9 pA Ib, RRIO, CMOS Operational Amplifier
OPA2156
25 MHz
Ultra-Low Noise, FET-Input, Precision RRIO Op-Amp
OPA1671
12 MHz
5 V, RRIO Low-Noise Audio Op-Amp
OPA325
10 MHz
Precision, 10 MHz, Low-Noise, Low-Power, RRIO, CMOS Zero-Crossover Operational Amplifier
OPA187
550 kHz
Zero Drift, Low-Power, Rail-to-Rail Output Amplifier
2
EMI-Hardened Op Amps Reduce Errors In Pulse Oximeters Soufiane Bendaoud
Copyright © 2019, Texas Instruments Incorporated
SNOAA31A – April 2019 – Revised August 2019
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