How to reduce the output noise of instrumentation amplifier

The use of an auto-zeroing noise filter actually reduces the instrumentation amplifier's output noise. Take the MAX4209 instrumentation amplifier as an example to provide a reference design for reference.

Auto-zero amplifiers have low voltage offsets, drift, and provide high gain and common-mode rejection ratios. However, this type of amplifier has a disadvantage: there is significant noise at the auto-zero frequency and its frequency multiplication. The auto-zero frequency is outside the effective bandwidth of a typical instrumentation amplifier. In some applications, the output of the instrumentation amplifier is directly connected to the analog-to-digital converter (ADC). These noises can directly affect the performance of the system.

Instrumentation Amplifiers Typical Application Instrumentation Amplifiers One of the most popular applications in medical systems is the Electrocardiogram Monitor (ECG), which monitors heart rate using sensors that are in contact with human skin. ECG sensors are used in pairs to detect very weak differential signals, typically only a few hundred microvolts to a few millivolts, with large offset voltages. For example, the offset voltage between the patient's left and right arms may reach 200mV. The differential ac signal is amplified by an instrumentation amplifier with a high dc common mode rejection ratio. The amplifying circuit also employs a high pass filter to eliminate the different dc components produced by different sensors.

Since instrumentation amplifiers are usually placed in the first stage of the entire amplification link, high input impedance and high CMRR are required. In addition, because the input differential signal is in the sub-millivolt level, the amplifier needs to provide high gain in the standard 0.05Hz to 150Hz bandwidth. The gain of the entire analog link is usually 1000 times. Therefore, the gain of the first-stage instrumentation amplifier is preferably in the range of 20 to 100. Given the high gain requirement, the input offset voltage (VOS) must be minimized to ensure adequate output dynamic range.

The suppression of 50Hz/60Hz noise from neighboring equipment and power lines is a basic requirement for ECG design. Therefore, the high CMRR and PSRR of the instrumentation amplifier at 50Hz/60Hz frequency becomes a key factor affecting the design. Finally, low-power devices with shutdown functionality are also basic requirements for many portable ECG system designs.

Indirect Current Feedback Architecture As a new instrumentation amplifier, Maxim's patented indirect current-feedback architecture* has some important advantages over the traditional three-op-amp architecture (Figure 1). For details on the indirect current feedback architecture, refer to the Maxim website.

As a new instrumentation amplifier, the indirect current feedback architecture has some important advantages over the traditional three-op-amp architecture (Figure 1). For details on the indirect current feedback architecture, see application note 4034: “Constraints on Instrumentation Amplifiers with Three Op Amp Architectures.”

Figure 1. Traditional three-op-amp instrumentation amplifier architecture. The resistance in the dashed line is the external resistance of the device.

Figure 2 shows the new indirect current feedback architecture that the MAX4209 uses.

Figure 2. The MAX4209 indirect current-feedback instrumentation amplifier. A and B in Figure 2 are two transconductance amplifiers that generate output current from their differential input voltage and reject the common-mode input signal. C is a high gain amplifier and provides negative feedback through R1 and R2. The negative feedback loop forces the two differential inputs of amplifiers A and B to be equal. Therefore, the relationship between the amplifier output and the differential input VIN is as follows:

VOUT=VIN×(1+R2/R1)

among them:

VIN=VIN+-VIN-

Compared with the traditional solution, this indirect current feedback architecture has two important advantages:

The input common-mode power supply is suppressed in the first stage, allowing the instrumentation amplifier to be powered from a single supply and to handle zero-potential or negative-potential detection over the entire gain range.

The amplifier gain is set by two internal matching resistors, greatly improving the gain accuracy.

Basic principle of auto-zero amplifier In order to continuously calibrate the offset voltage of the amplifier, the auto-zero amplifier uses a “zero” amplifier in parallel with the signal path, and the internal oscillator operates at an auto-zero frequency (fC), typically several tens of kHz. . The work process is divided into two phases, as shown in Figure 3. Auto-zero phase: Both switches are set to 1, and the capacitor (C1) is charged to the offset voltage of the nulling amplifier (A2). The offset voltage of the main amplifier (A1) (held by C2) is calibrated by the NULL pin. Amplification phase: Both switches are set to position 2; C1 holds the offset voltage of the nulling amplifier (already calibrated with the NULL pin); the offset voltage of A1 is measured by A2 and stored on C2.

Figure 3. Basic operation of the auto-zero amplifier. The auto-zero amplifier forms a data sampling system, which results in a sum or difference of the sampling or auto-zero frequency fC and the signal frequency (fS). To avoid aliasing distortion, the signal bandwidth is limited to less than half of fC.

Automatic zeroing allows the amplifier to greatly reduce the input offset voltage, VOS, to a few microvolts with an offset voltage drift of up to one-several microvolts per degree Celsius. If fC is much higher than the noise cutoff frequency, 1/f noise can still be suppressed. In theory, the auto-zero amplifier does not have 1/f noise, but the chopping operation increases the output white noise over a wider frequency band.

Reduce Noise Near Auto-Zero Frequency The MAX4209 is an indirect current-feedback instrumentation amplifier with very high dc accuracy due to the built-in auto-zero circuit. In some applications, the output of the MAX4209 is directly connected to the ADC, filtering its output noise to improve system performance. The output noise is composed of broadband white noise and the auto-zero frequency fC and the burrs at its frequency multiplication. Especially when the difference between the sampling frequency of the ADC and the auto-zero frequency fC falls on the effective frequency band, it is more important to introduce this filter.

The test results presented in this paper use the MAX4209H with a fixed gain of 100. The device's signal bandwidth is 7.5kHz and fC is approximately 45kHz. The first-order low-pass filter is formed by the external capacitor (C) placed between the OUT and FB pins of the amplifier in parallel with the internal resistor (R2). The pole of the filter is determined by C and R2. The internal R2 of the MAX4209 is 99kΩ. 4 is a noise measurement circuit.

Designers need to trade off the required noise suppression and signal bandwidth limitations based on the application. The following table summarizes the trade-offs in the absence of external capacitors, C = 1nF, and C = 10nF.

In some applications, if there is a higher requirement for noise suppression, more external filter components than the feedback capacitor can be used, and connecting a simple RC low-pass filter at the output of the amplifier can provide higher noise attenuation. Figure 7 and Figure 8 show the input reference noise curve with RL=39Ω and CL=760nF as output low-pass filters. Corresponding to these component values, the RC filter's pole is near 5 kHz and provides about 18 dB of attenuation at the 45 kHz auto-zero frequency fC.

For applications that amplify weak input signals at high common-mode voltages, the instrumentation amplifier must maintain extremely low offset voltage, drift, and very high gain accuracy, as well as high CMRR. Auto-zeroed indirect current feedback amplifiers can meet these performance requirements but increase output noise. This article uses a very simple method (ie adding an external capacitor or up to 3 external components) to effectively reduce the noise of the indirect current feedback amplifier MAX4209.