Hello everyone, I am Rose. Welcome to the new post today. Today I will compare two amplifier to you. Differential amplifier and current sense amplifier.

Ⅰ.  High-side current detection

In many power electronic systems, such as motor control, coil drive, and power management, it is necessary to detect the positive output current of the power supply  (also known as high-side current detection: High-Side Current Sensing) (such as DC-DC  conversion, battery detection, etc.). Current sensing can be improved in these applications by measuring the current on the positive (high side) of the power supply rather than the negative side (ie, the current return side). The short-circuit current to the ground, for example, can be determined, as can the current in the freewheeling diode. Obtaining supply current via a shunt on the negative side of the power supply may result in ground potential discrepancies. Figure 1 and Figure 2 below show the circuit configuration using the high side to sense the motor and solenoid currents.

Figure. 1 High-Side Current Detection in the Solenoid Drive Circuit

Figure. 2 High-Side Current Detection Circuit of H-Bridge Motor Drive Circuit

Figure. 3 Three-Phase Motor Drive High-End Current Detection  

When  PWM  drive is utilized in the above three current-sense applications, the common-mode voltage across the current-sense resistor swings from 0V to the battery voltage. The power field-effect transistor in the circuit generates this  PWM input signal, which has a periodic, high frequency, quick rise and fall characteristic. As a result, op amps for signal processing of high-side current shunts must be capable of having exceptionally strong common-mode rejection, high gain, high precision, and low (voltage, current) bias at the same time.

The current of the MOS FET drive coil always flows from top to bottom in the electromagnetic coil drive circuit depicted in Figure 1, allowing the unidirectional current detection to match the criteria. However, because the current in the motor driving circuits shown in Figures 2 and 3 is bidirectional, the circuit must be capable of handling both positive and negative current signals.

Many semiconductor firms now offer alternative chips for boosting high-side current sensing, which designers may find useful. One of the most significant things to keep in mind is that all alternative current detection IC chips may be split into two categories: current detection amplifier chips and differential amplifier chips.

To assist electronic engineers in selecting the most appropriate high-side current sensing solution for application needs, we will highlight and explain the key distinctions between the following two types of signal processing chips.The bidirectional differential high-voltage operational amplifier AD8206  [3] and the bidirectional current-sense amplifier AD8210[4] are compared below. Although these two op amps share the same external pins and can both be used for high-side current sensing, their performance and internal construction differ. So, in terms of practical application, which one should we choose?

Ⅱ.  Basic working principles

The AD8206  integrated high-voltage differential amplifier can handle a power wave voltage of up to 65V, as shown in Figure 4. To limit the common-mode voltage to the input voltage range of op amp A1, a 16.7:1 back-voltage resistor is used at the chip input. Unfortunately, the input voltage divider also attenuates the differential signal correspondingly, thus the overall voltage gain of 20V/V may be achieved using the 344V/V voltage gain provided by the A1 and A2 stages.

Figure. 4 AD8206 Simplified Schematic

A positive reference voltage can be established for the positive input of the output amplifier A2 in the AD8206  using a low-impedance reference voltage source to achieve bidirectional current sensing. Even if the common mode voltage is negative, the chip can continue to amplify the voltage signal across the current shunt resistor.

Figure 5 depicts the AD8210 high-voltage current sensor amplifier circuit, which was recently released. It has a similar function to the AD8206  and uses the same pin designations, but it has a distinct working principle and uses different technology. index.

Figure. 5 AD8210 Internal Function Diagram

The most significant difference is that the AD8210 does not use an attenuation resistor network to lower the high power wave voltage at the input. The XFCB IC manufacturing process generates a high-voltage triode for its input terminal. It can handle voltages up to 65V because the corresponding VCE can be as high as 65V. Input voltage in male mode. Figure 5 shows how the AD8210 amplifies the tiny current differential signal. Through R1 and R2, the chip's positive and negative ends of the first amplifier A1 are linked to the two ends of the current sampling resistor. A1 directs current via transistors Q1 and Q2 to balance the voltage at the positive and negative input terminals. On the internally exactly matched resistors (there is no common mode voltage), the on-currents of Q1 and Q2 form a proportional voltage, which is amplified and output by amplifier A2. The output voltage is 20:1 in relation to the input differential voltage, and A2 is driven by +5V. The input structure of the AD8210 current amplifier demands that the input signal power wave voltage be larger than 2V or 3V, and that it cannot be less than 0. The AD8210's built-in pull-up resistor boosts the A1 input voltage such that the input common-mode voltage can be as low as -2V.

Ⅲ.  Differences between the two chips

The functioning mechanisms of the current sense amplifier (AD8210) and the differential amplifier (AD8210) are obviously different (AD8206). The former involves converting the input differential signal into distinct currents to ground, followed by converting the chip's internal resistance into a differential signal without common-mode voltage and sending out a big output via the post-stage operation. To resist common-mode high voltage, the chip primarily relies on high-voltage semiconductor technology. The latter attenuates the signal uniformly with an input attenuation resistor network, then amplifies the differential signal in the input signal with differential amplification. To reduce the common mode high voltage, the chip uses a resistor network.

Although the two chips' key performance metrics have been discussed in their datasheets, some discrepancies based on internal structural differences are not immediately visible. Some critical elements to consider when designing the optimum solution are listed below.

1. Amplifier bandwidth

The frequency response bandwidth of the current sense amplifier is usually only approximately one-fifth of the frequency response bandwidth of the differential amplification technique due to the attenuation of the input signal. Despite this, the bandwidth of these two chips is adequate for the majority of applications.

In the magnetic drive, for example, a PWM drive with a frequency of more than 20kHz is frequently necessary. In order to account for noise, the current signal amplification bandwidth must be bigger than 20kHz. Because the average current stability is frequently addressed in magnetic control, signal bandwidth requirements are not large. A greater current amplification bandwidth is necessary for current sampling in motor control, particularly for current clockwise current acquisition under PWM signal management. Instead of AD8206.  a current sense amplifier (AD8210) should be considered at this time. It has the ability to provide a more accurate current waveform for the current signal.

Figure. 6 Current Waveform and Voltage Waveform Output by AD8206

2. Common mode rejection ratio

Current amplifiers can provide improved common-mode voltage rejection (CMR:  Common-Mode Rejection ) performance for common-mode voltage rejection. Internally perfectly matched high-voltage transistors like the AD8210, for example, can deliver up to 100 dB CMR. The CMR of the AD8206.  which relies on the attenuation resistor network, is roughly 80 dB because it can only attain 0.01 percent accuracy.

3. External filter network influence

An  RC  low-pass filter is added to the input end of the amplifier circuit to decrease current noise.  Rf and Cf, for example, are employed to create a low-pass filter for the current signal in the diagram below.

Figure. 7 Input Filter Network

The input resistance impedance of a differential mode amplifier is more than 100k. The input resistance of the AD8206.  for example, is 200k. The gain error is roughly 0.1 percent if the external current filter resistance  Rf is 200. If the matching error between the two low-pass filter resistors Rf is also around 1%, the CMR effect will be around 94 dB, which is not much higher than the device's 80 dB.

However, it has a very high common mode input resistance for current-sensing amplifiers. However, the input resistance Rin of the amplifier is only about 5k in order to transform the input differential voltage into differential current. The Rin of the AD8210, for example, is 3.5k ohms. This external low-pass filter causes a gain inaccuracy of up to 5.4 percent! The CMR is also decreased to 59 dB at the same time. When employing a current amplifier, extra attention should be paid to the external low-pass filter network's specifications, such as the filter resistance, which should ideally be less than 10 ohms.

4. Input overload

If the load has excessive voltage and current, it can generate a large differential voltage across the current sensing amplifier AD8210, which can harm the chip. The AD8206  with differential amplification offers a greater tolerance range for the load's overcurrent and overvoltage, and chip collapse is difficult to achieve.

5. Reverse voltage protection

The equipment supply voltage may be reversed in some situations, resulting in a reproduced extremely high negative common mode voltage across the current amplifier. The infrequent negative common-mode voltage is tolerated well by a differential amplifier (AD8206) with a voltage divider resistor network input, but the situation is significantly worse with the AD8210. Because the chip's input Rin resistance is low, a high negative common mode voltage will force the chip's ESD diode to conduct, causing damage to the internal circuit.

6. Input bias current

The chip's quiescent operating current must be considered in some low-power application circuits. Even when the chip is not powered, the AD8206  's input resistor network uses high-side supply current. After the circuit is shut down, the associated AD8210 will likewise turn off the internal transistor circuit, consuming very little power supply current. As a result, the AD8210 may be better suited to battery-powered low-power applications.

Ⅳ.  Summary of current detection scheme

Electric vehicles, communications, consumer products, and industrial applications all require high-end current sensing. The design can use either a detection strategy based on differential voltage amplification or a detection scheme based on current sensing. Despite the fact that these ICs have the same function and pin definitions, when high acquisition accuracy and system dependability are required, it is vital to consider and select an acceptable current detecting method based on the two schemes' differing internal mechanisms. The table below shows a comparison of the two choices.

What are the four input and output methods of differential amplifiers?

There are four input and output modes of differential amplifier: double-ended input, single-ended input, double-ended output, and single-ended output.

What is the difference between a differential amplifier and a differential amplifier？

The differential amplifier is in the form of a symmetrical tube, and the differential mode signal and the common mode signal can be.

The differential amplifier has no common mode signal, only amplifies the differential mode signal, and does not need to consider Avc when calculating

Figure. 8 Comparing Current Amplification and Differential Amplification Schemes