Electrocardiogram (ECG) Front End
Biophysical Monitoring : Electrocardiogram (ECG) Front End
LOW OFFSET: 100 µV (max)
LOW OFFSET DRIFT: 0.4 µV/°C (max)
EXCELLENT LONG-TERM STABILITY
VERY-LOW 1/f NOISE
TRUE RAIL-TO-RAIL I/O
INPUT COMMON-MODE RANGE:
20mV Below Negative Rail to 100mV Above Positive Rail
WIDE OUTPUT SWING: Within 10mV of Rails
SUPPLY RANGE: Single +2.7V to +5.5V
microPACKAGE: MSOP-8, MSOP-10
LOW-LEVEL TRANSDUCER AMPLIFIER FOR BRIDGES, LOAD CELLS, THERMOCOUPLES
WIDE DYNAMIC RANGE SENSOR MEASUREMENTS
HIGH-RESOLUTION TEST SYSTEMS
MULTI-CHANNEL DATA ACQUISITION SYSTEMS
The INA326 and INA327 (with shutdown) are high-performance,
low-cost, precision instrumentation amplifiers with rail-to-rail input and output.
They are true single-supply instrumentation amplifiers with very low DC errors
and input common-mode ranges that extends beyond the positive and negative rails.
These features make them suitable for applications ranging from general-purpose to high-accuracy.
Excellent long-term stability and very low 1/f noise assure low offset voltage
and drift throughout the life of the product.
The INA326 (without shutdown) comes in the MSOP-8 package.
The INA327 (with shutdown) is offered in an MSOP-10.
Both are specified over the industrial temperature range, –40°C to +85°C,
with operation from –40°C to +125°C.
★ INA321 의료 측정용 증폭기
CMOS 계측기용 증폭기로 정밀신호 포착 구현
다양한 정밀 계측기용 증폭기 제품군으로 비용절감, 설계시간 단축,
기판면적 감소, 조립공정 단순화, 신뢰도 향상 실현
Texas Instruments는 초절전형 rail-to-rail 출력기능의 CMOS 계측기용 증폭기를 발표했다.
INA321로 명명된 이 제품의 기능은 비용에 매우 민감한, 대량생산을 주로 하는 가전, 통신장비, 컴퓨터,
자동차업체는 물론 산업용 공정제어 및 의료장비 업체에서 이상적으로 활용할 수 있다.
제품에 대한 좀더 자세한 정보는 http://www.ti.com/sc/aap5251u 에서 참고할 수 있다.
INA321과 같은 계측기용 증폭기는 정밀신호 포착용 증폭기의 벤치마크 제품이다.
이 제품은 전기잡음이 심한 환경에서 장애로부터 차동신호를 추출하여 측정 정확도를 확보한다.
대기 전류의 40㎂에서 동작하는 INA321은 저비용, 저전력 증폭이 가능하다.
또한 배터리 전원 시스템에서는 셧다운시 성능의 최적화를 위해 1㎂ 미만의 전류를 유지한다.
수 ns 이내에 정상동작 상태로 전환될 수 있기 때문에 전원관리 기능도 내장하고 있다.
TI의 Burr-Brown 계측기 전략 마케팅 엔지니어 존 브라운(John Brown)은
"INA321은 CMOS 기술과 저가격, rail-to-rail 출력스윙, 낮은 입력 바이어스 전류등 우수한 기능을 결합하였기 때문에
업계 최고인 우리의 계측기용 증폭기 제품군이 한층 강화될 것"이라면서
"INA321에 대한 도전은 오히려 더 적은 비용으로 이 같은 기능을 구현할 수 있다고 생각하는 고객들로부터 제기되는 셈이다.
그러나 고정밀도, 저가격, 소형 패키징 등의 특성으로 $1 수준의 가격대 제품이면서도
높은 신뢰성과 성능을 구현해야 하는 설계자들에게는 최적의 제품인 셈이다.
게다가 설계소요시간을 단축해 주고 보드 면적을 줄여주며
대량신호 수집 시스템의 조립공정을 단순하게 한다."고 말했다.
저전력 설계임에도 불구하고 대역폭(500kHz, G=5)이 손상되지 않는 독특한 점 때문에
INA321은 일반 용도는 물론 샘플링 A/D 컨버터 구동용으로도 이상적인 제품이다.
주요 사양으로는 rail-to-rail 출력스윙(25mV), 저바이어스 전류(10pA), 높은 동상분 제거(94dB),
저 오프셋 전압(+/-200㎶) 등을 들 수 있다. 내부 설정된 5의 이득(이득 정확도 0.02%)과
더 높은 이득 설정을 위한 사용자 프로그래밍 외부 이득 저항기를 갖춰 융통성도 매우 높다.
★Signal Acquisition Challenges
The action potential created by heart wall contraction spreads electrical currents from the heart throughout the body. The spreading electrical currents create different potentials at different points on the body, which can be sensed by electrodes on the skin surface using biological transducers made of metals and salts. This electrical potential is an AC signal with bandwidth of 0.05 Hz to 100 Hz, sometimes up to 1 kHz. It is generally around 1-mV peak-to-peak in the presence of much larger external high frequency noise plus 50-/60-Hz interference normal-mode (mixed with the electrode signal) and common-mode voltages (common to all electrode signals).
The common-mode is comprised of two parts: (1) 50- or 60-Hz interference and (2) DC electrode offset potential. Other noise or higher frequencies within the biophysical bandwidth come from movement artifacts that change the skin-electrode interface, muscle contraction or electromyographic spikes, respiration (which may be rhythmic or sporadic), electromagnetic interference (EMI), and noise from other electronic devices that couple into the input. Some of the noise can be cancelled with a high-input-impedance instrumentation amplifier (INA), like the INA326 or INA118, which removes the AC line noise common to both inputs and amplifies the remaining unequal signals present on the inputs; higher IA CMR will result in greater rejection. Because they originate at different points on the body, the left-arm and right-arm ECG signals are at different voltage levels and are amplified by the IA. To further reject 50- and 60-Hz noise, an operational amplifier deriving common mode voltage is used to invert the common-mode signal and drive it back into the patient through the right leg using amplifier A2. Only a few microamps or less are required to achieve significant CMR improvement and stay within the UL544 limit.
★Three ECG electrodes connected to patient using CMOS devices with 5-V single supply.
As in most other applications, the system supply voltage in biophysical monitoring continues the trend toward low, single-supply levels. While bipolar supplies are still used, 5-V systems are now common and trending to single 3.3-V supplies. This trend presents a significant challenge for the designer faced with a 500-mV electrode potential and emphasizes the need for a precision signal conditioning solution. While the following discussion concentrates on the single supply design, the principles involved apply to bipolar designs as well. A list of recommended single and bipolar supply devices can be found below.
★ 주파수 응답특성
Standard -3-dB frequency for patient monitoring is 0.05 Hz to 30 Hz, while diagnostic grade monitoring requires 0.05 Hz to 100 Hz or more. All ECG front ends must be AC coupled to remove artifacts from the electrode offset potential, though important features of the ECG waveform have extremely low frequency characteristics.
★ 전극의 전위 (Electrode Potential)
Because electrode potential can reach +/-500 mV, eliminating the effects of electrode potential by AC coupling at low frequency allows for precise measurements. A DC restorator amplifier in a feedback configuration nulls out the DC offset. If the left arm DC offset is +300 mV and the right arm electrode is 0-V DC, the differential input voltage is 300 mV. Because the instrumentation amp has a gain of 10, 3 V appears at the output of the instrumentation amp. With a gain of 50 or more, the output amplifier would try to drive the signal up to 150 V but never does because a feedback integrator applies an equal negative voltage to the reference point. Using this linear summing effect, the 3-V positive offset is cancelled by the negative 3-V correction voltage. The result of this DC restorator is to turn the original DC-coupled amplifier into an AC-coupled amplifier. Because the DC electrode offset has been removed, the output stage can amplify the signal to maximize the data converter input range without becoming saturated.
★ 측정용 증폭기 요구사항
Stability in low gain (G = 1 to 10)
High common-mode rejection (CMR)
Low input bias current (IB)
Good swing to the output rail
Very low offset and drift
Operational Amplifier Requirements
Low noise in high gain (Gain = 10 to 1000)
Very low offset and drift
The INA321 is a modified version of the classic two op amp instrumentation amplifier, with an additional gain amplifier. Figure 1 shows the basic connections for the operation of the INA321 and INA2321. The power supply should be capacitively decoupled with 0.1µF capacitors as close to the INA321 as possible for noisy or high-impedance applications. The output is referred to the reference terminal, which must be at least 1.2V below the positive supply rail.
The INA321 family is fully specified over a supply range of +2.7V to +5.5V, with key parameters assured over the temperature range of −55°C to +125°C. Parameters that
vary significantly with operating conditions, such as load conditions or temperature, are shown in the Typical Characteristics. The INA321 may be operated on a single supply.
Figure 2 shows a bridge amplifier circuit operated from a single +5V supply. The bridge provides a small differential voltage riding on an input common-mode voltage.
★ 증폭도의 설정
The ratio of R2 to R1, or the impedance between pins 1, 5, and 6, determines the gain of the INA321. With an internally set gain of 5, the INA321 can be programmed for gains greater than 5 according to the following equation:
G = 5 + 5 (R2/R1)
The INA321 is designed to provide accurate gain, with gain error assured to be less than 0.1%. Setting gain with matching TC resistors will minimize gain drift. Errors from external resistors will add directly to the gain error, and may become dominant error sources.
★INPUT COMMON-MODE RANGE
The upper limit of the common-mode input range is set by the common-mode input range of the second amplifier, A2, to 1.2V below positive supply. Under most conditions, the amplifier operates beyond this point with reduced performance. The lower limit of the input range is bounded by the output swing of amplifier A1, and is a function of the reference voltage according to the following equation:
VOA1 = 5/4 VCM — 1/4 VREF
(See Typical Characteristics for Input Common-Mode Range vs Reference Voltage).
The reference terminal defines the zero output voltage level. In setting the reference voltage, the common-mode input of A3 should be considered according to the following equation:
VOA2 = VREF + 5 (VIN+ − VIN−)
For optimal operation, VOA2 should be less than VDD − 1.2V.
The reference pin requires a low-impedance connection. As little as 160Ω in series with the reference pin will degrade the CMRR to 80dB. The reference pin may be used to compensate for the offset voltage (see Offset Trimming section). The reference voltage level also influences the common-mode input range (see Common-Mode Input Range section).
★INPUT BIAS CURRENT RETURN
With a high input impedance of 1013Ω, the INA321 is ideal for use with high-impedance sources. The input bias current of less than 10pA makes the INA321 nearly independent of input impedance and ideal for low-power applications. For proper operation, a path must be provided for input bias currents for both inputs. Without input bias current paths, the inputs will float to a potential that exceeds common-mode range and the input amplifier will saturate.
Figure 3 shows how bias current path can be provided in the cases of microphone applications, thermistor applications, ground returns, and dc-coupled resistive bridge applications.
Figure 3. Providing an Input Common-Mode Path When differential source impedance is low, the bias current return path can be connected to one input. With higher source impedance, two equal resistors will provide a balanced input. The advantages are lower input offset voltage due to bias current flowing through the source impedance and better high-frequency gain.
The INA321 is optimized for a load impedance of 10kΩ or greater. For higher output current the INA321 can be buffered using the OPA340, as shown in Figure 4. The OPA340 can swing within 50mV of the supply rail, driving a 600Ω load. The OPA340 is available in the tiny MSOP-8 package.
The shutdown pin of the INA321 is nominally connected to V+.
When the pin is pulled below 0.8V on a 5V supply,
the INA321 goes into sleep mode within nanoseconds.
For actual shutdown threshold, see the Typical Characteristic curve,
Shutdown Voltage vs Supply Voltage.
Drawing less than 1µA of current, and returning from sleep mode in microseconds,
the shutdown feature is useful for portable applications. Once in sleep-mode,
the amplifier has high output impedance, making the INA321 suitable for multiplexing.
★ RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is used to
achieve rail-to-rail output for gains of 10 or greater.
For resistive loads greater than 25kΩ, the output voltage can swing to
within a few millivolts of the supply rail while maintaining low gain error.
For heavier loads and over temperature, see the Typical Characteristic curve,
Output Voltage Swing vs Output Current.
The INA321’s low output impedance at high frequencies makes it suitable
for directly driving Capacitive Digital-to-Analog (CDAC) input A/D converters, as shown in Figure 5.
★ 영점오차(OFFSET ) 조정
The INA321 is laser-trimmed for low offset voltage. In the event that external offset adjustment is required,
the offset can be adjusted by applying a correction voltage to the reference terminal.
Figure 6 shows an optional circuit for trimming offset voltage.
The voltage applied to the REF terminal is added to the output signal.
The gain from REF to VOUT is +1.
An op-amp buffer is used to provide low impedance
at the REF terminal to preserve good common-mode rejection.
★ 입려회로의 보호
Device inputs are protected by ESD diodes that will conduct if the input voltages exceed the power supplies by more than 500mV. Momentary voltages greater than
500mV beyond the power supply can be tolerated if the current through the input pins is limited to 10mA. This is easily accomplished with input resistor RLIM, as shown in Figure 7. Many input signals are inherently current-limited to less than 10mA; therefore, a limiting resistor is not required.
★OFFSET VOLTAGE ERROR CALCULATION
The offset voltage (VOS) of the INA321E is specified at a maximum of 500µV with a +5V power supply and the common-mode voltage at VS/2. Additional specifications for power-supply rejection and common-mode rejection are provided to allow the user to easily calculate worst-case expected offset under the conditions of a given application.
Power-Supply Rejection Ratio (PSRR) is specified in µV/V. For the INA321, worst-case PSRR is 200µV/V, which means for each volt of change in power supply, the offset may shift up to 200µV. Common-Mode Rejection Ratio (CMRR) is specified in dB, which can be converted to µV/V using the following equation:
CMRR (in µV/V) = 10[(CMRR in dB)/ -20] •* 10^6
For the INA321, the worst-case CMRR over the specified common-mode range is 90dB (at G = 25) or about 30µV/V. This means that for every volt of change in common-mode,
the offset will shift less than 30V. These numbers can be used to calculate excursions from the specified offset voltage under different application conditions. For example, an application might configure the amplifier with a 3.3V supply with 1V common-mode. This configuration varies from the specified configuration, representing a 1.7V variation in power supply (5V in the offset specification versus 3.3V in the application) and a 0.65V variation in common-mode voltage from the specified VS/2.
Calculation of the worst-case expected offset would be as follows:
Adjusted VOS = Maximum specified VOS + (power-supply variation) * PSRR +
(common-mode variation) * CMRR
VOS = 0.5mV + (1.7V * 200µV) + (0.65V * 30µV) = ±0.860mV
However, the typical value will be smaller, as seen in the Typical Characteristics.
★ 되먹임 정전용량과 개선된 응답특성
For optimum settling time and stability with high-impedance feedback networks,
it may be necessary to add a feedback capacitor across the feedback resistor, RF, as shown in Figure 8.
This capacitor compensates for the zero created by the feedback network impedance
and the INA321’s RG-pin input capacitance (and any parasitic layout capacitance).
The effect becomes more significant with higher impedance networks.
Also, RX and CL can be added to reduce high-frequency noise.
★Figure 8. Feedback Capacitor Improves Dynamic Performance
It is suggested that a variable capacitor be used for the feedback capacitor since input capacitance may vary between instrumentation amplifiers, and layout capacitance is difficult to determine. For the circuit shown in Figure 8, the value of the variable feedback capacitor should be chosen by the following equation:
RIN * CIN = RF * CF
where CIN is equal to the INA321’s RG-pin input capacitance (typically 3pF) plus the layout capacitance. The capacitor can be varied until optimum performance is obtained.
★ Figure 9 shows the INA321 configured to serve as a low-cost ECG amplifier,
suitable for moderate accuracy heart-rate applications such as fitness equipment.
The input signals are obtained from the left and right arms of the patient.
The common-mode voltage is set by two 2MΩ resistors.
This potential, through a buffer, provides anoptional right leg drive.
Filtering can be modified to suit application needs by changing the capacitor value of the
★ Low-Power, Single-Supply Data Acquisition Systems
Refer to Figure 5 to see the INA321 configured to drive an ADS7818.
Functioning at frequencies of up to 500kHz,
the INA321 is ideal for low-power data acquisition.
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