Use Spice to analyze DRL in an ECG front end
Understand this critical analog front end for this ubiquitous, vital ECG medical instrument.
Matthew Hann, Texas Instruments -- EDN, January 5, 2012
ECG (electrocardiography) is the science of converting the ionic depolarization of the heart to a measurable electrical signal for analysis. One of the most common challenges in the design of analog electronics interfaces to the electrodes or to patients is optimization of the DRL (driven-right-leg) circuit, which is often added to biological-signal amplifiers to reduce common-mode interference and to increase performance and stability. Using Spice to help in this effort can greatly simplify the process.
In an ECG front end, the DRL amplifier provides a common
electrode bias at the reference voltage, VREF, and feeds
back the inverted common-mode noise signal, eNOISECM, to
reduce the overall noise seen at the inputs of the instrumentation
amplifier’s gain stage. The positive and negative
ECG sources, ECGP and ECGN, are split to show how the
DRL amplifier provides the common reference point for
a portion of the ECG signal that is seen at the positive and the negative inputs of the instrumentation amplifier
(Figure 1).The parallel RC (resistance/capacitance) combination for the left arm, the right arm, and the right leg represents the lumped passive-electrode connection impedances, which are 52 kΩ and 47 nF. Assuming that eNOISE couples parasitically into the inputs, the feedback of eNOISECM will reduce the overall noise signal at each input, leaving the task of either externally filtering the residual noise or having the CMRR reject the instrumentation amplifier’s common-mode noise.
Figures 2, 3, and 4 show the variation in CMRR of the
common-mode test circuit with the varying gain of the DRL
amplifier. These plots show that you can achieve the best low-frequency
CMRR with no feedback resistor, yielding infinite
gain. In reality, however, eliminating the dc path, setting
RF to a high value, or using both of these methods may be
impractical for applications that require the linear operation of the DRL amplifier when one of the input amplifier’s leads
is removed.
Once you determine the gain of the DRL amplifier, the
next step is to inject a small signal step in the loop and monitor
the output response (Figure 5). In this case, the response
shows a strong output oscillation, indicating instability in the loop (Figure 6). The dominant feedback path causing this
instability is the feedback path for the body, the electrode,
and the instrumentation-amp feedback path around the DRL
amplifier. A test circuit allows you to separate and analyze the
feedback and the open-loop-gain curve of the DRL amplifier
on a bode plot (Figure 7).
Without an external compensation network, the beta-distribution
curve approaches the open-loop-gain curve at
a rate of closure greater than 20 dB per decade, indicating
instability. To address this issue (Figure 8), add series resistor
RC and capacitor CC (Figure 9) in the local feedback of
the DRL amplifier. ZC then becomes the dominant feedback
path between 20 and 30 kHz. The result for the simulation
in Figure 7 is represented by the beta (feedback) curve in
Figure 10. Figure 11 shows the full circuit of the DRL with
compensation. Figure 12 shows the compensated beta-curve
plots, employing variations in RC and CC. The overall beta
curve intersects the open-loop-gain curve with a rate of closure
that is 20 dB per decade or less and a loop gain with a
phase margin greater than 45° (Figure 13).
Acknowledgment
This article originally appeared on EDN’s sister site, Planet Analog.
Author’s biography
Matthew William Hann is a precision-analog-applications manager at Texas Instruments. He has more than a decade of product expertise, which includes temperature sensors, difference amplifiers, instrumentation amplifiers, programmable-gain amplifiers, power amplifiers, and TI’s line of ECG analog-front-end devices. Through his role as an applications engineer, Hann has developed a focused expertise on the design of analog front ends for medical applications, such as ECGs, electroencephalograms, electromyograms, blood-glucose monitoring, and pulse oximetry. Hann received a bachelor’s degree in electrical engineering from the University of Arizona—Tucson. You can reach him at ti_ matthann@list.ti.com.
Talkback
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In three electrode biosignal sensing technique, the purpose of the third (reference) electrode is to provide a low impedance return path for the flowing via body interference currents.
Usually the third electrode is connected to the front-end signal ground or is closed in a control loop, the so called DRL circuit. When the third electrode is directly connected to signal ground, the flowing interference current multiplied by the third electrode impedance is converted to a voltage drop which is appeared as an amplifier common mode input voltage. If the third electrode is connected to the DRL circuit, it action is drive the third electrode by the opposite voltage in order to reduce the common mode voltage at the amplifier inputs.
So, the DRL circuit is a circuit with a common mode shunt-shunt negative feedback (parallel to input voltage feedback) which reduces the impedance of the third electrode by its feedback factor F=(Adrl+1), where Adrl is the DRL loop gain.
The DRL circuit should have as much as possible loop gain especially for low-frequency, i.e. for 50Hz/60Hz power-line interference. The DRL circuit simply is an opamp integrator stage.
The human body usually has a stray capacitance to the earth in range from 100pF to 200pF, also the most of the present day biosignal amplifiers are isolated. Thus, the body-earth stray capacitance or the isolated ground capacitance is the first major factor for DRL instability because they introduce an additional pole in the DRL control loop!
Dobromir Dobrev - 2012-12-1 12:23:33 PST -
Most ECG front ends are galvanically isolated. in which case, what the DRL circuit really does is to drive common terminal of the isolated circuits to the potential of RL lead. The current sourced by the DRL output amplifier comes from the isolated power supply of the front end. The inverting input of the amplifier is connected to the isolated common. the circuit causes causes the circuit common of the front end to to approximate the common mode voltage. Fortunatly, if the impedance from the isolated front end to the patient is very high--and for safety it should be--you can neglect this inthe simulation. Just remember to keep it in mind if the circuit doesn't work as expected.
If you do try to model the real situation, the Spice model for the amplifier must accuratly represent the current drawn from the power supply--most do not. The impedance from the common of the front end to the patient must be included in the model. Alternativly you can just add some impedance from the RL lead to ground of the model.
David A. Soss - 2012-10-1 11:10:54 PST
