High-side current sensor AD8205 and its application circuit design

AD8205 is a single-supply high-performance differential amplifier launched by Analog Devices, Inc., USA . The typical single-supply voltage is 5V, and its common-mode voltage input range is -2~65V. It can withstand input common-mode voltages of -5~+70V, and is suitable for industrial equipment that detects small differential voltages under high common-mode voltage conditions. Its gain is fixed at 50V/V, and its operating temperature range is -40~+125℃. The offset voltage temperature drift is less than 15mV/℃, and the gain temperature drift is less than 30ppm/℃ (the ambient temperature can be as high as 125℃). It has excellent DC performance within the entire specified temperature range, and has a common-mode rejection ratio of up to 80dB in the frequency band from DC to 100kHz. Therefore, its measurement loop error is small and the accuracy is high, which is very suitable for control systems such as motor control, transmission control, magnetic suspension control, vehicle power control, fuel injection control, engine management and DC-DC conversion.

Internal circuit structure and its working principle

The internal circuit of AD8205 consists of two integrated operational amplifiers A1 and A2, a resistor network, a small reference voltage source and a bias circuit. Its circuit structure is shown in Figure 1.

The pre-attenuation of A1 is composed of resistors RA, RB, and RC, which can attenuate the common-mode voltage to a suitable input voltage range. Two groups of attenuators form a bridge network with an attenuation rate of 1/16.7. After the input signal is attenuated, the amplitude of the input signal is kept within the power supply voltage range. When the input voltage exceeds the power supply voltage or is lower than the voltage of the common ground terminal, the internal reference voltage takes effect, so that the amplifier can still work normally when the negative common-mode voltage signal is input. When the bridge is balanced, the differential input signal generated by the common-mode voltage signal is 0V. Of course, the input network also attenuates the input differential voltage signal. Amplifier A1 amplifies the attenuated signal by 26 times, and its input and output are both differential to obtain the maximum AC common-mode rejection ratio. In addition, the resistance matching rate of resistors RA, RB, RC, RD and RF after laser calibration is better than 0.01%. This high-precision calibration enables the device to obtain a common-mode rejection ratio of more than 80dB.

Amplifier A2 converts the differential signal output by A1 into a single-ended signal and amplifies it by 32.15 times. Both reference input terminals VREF1 and VREF2 are connected to the in-phase input terminal of A2 through resistor RREF, so that the output can be adjusted to the required output voltage range. When the two reference input terminals are used in parallel, the gain of the reference voltage from input to output is 1V/V; when any reference input terminal is used alone, its gain is 0.5V/V. The total gain of AD8205 is composed of the attenuation rate of the attenuation circuit 1/16.7, the amplification factor of A1 26 and the amplification factor of A2 32.15. AD8205 has a pull-down current absorption capability of 300μA and uses a Class A PNP tube connected to a pull-up resistor for output.

Output mode settings

Unipolar output

This method is generally used to measure the current flowing through the sampling resistor in a unidirectional manner. There are two basic modes: ground reference and V+ reference output mode. In the unipolar operating mode, when the differential input is 0, the output can be biased to the negative direction (close to ground) or the positive peak (V+). When the differential voltage is applied to the input, the output will move in the opposite direction to the peak. At this time, the input differential voltage amplitude corresponding to the full-scale output is close to 100mV, and its polarity is determined by the static setting of the output voltage. When biased to the positive peak, the input differential voltage should be negative, and the output drops from the positive peak; conversely, if it is statically biased to ground, the input differential voltage should be positive and the output rises from 0.

The ground-referenced output connection is shown in Figure 2(a). Both of its reference inputs are connected to the ground, and when the input differential voltage is 0, its output is biased to the inverting peak (about 0.05V).

The output connection method with V+ as reference is shown in Figure 2(b). Both of its reference input terminals are connected to the positive power supply, and when the input differential voltage is 0, its output is biased to the positive phase peak (about 4.8V).

[page] Bipolar Output

In bipolar output, AD8205 can measure the bidirectional current flowing through the sampling resistor. At this time, the output can be biased to any position within the output range. When the current in the positive and negative directions being detected is of equal amplitude, its output must be biased to the middle position of the full-scale output. When the bidirectional current amplitude is asymmetric, the output bias can deviate from the half-scale position accordingly. Its two reference inputs VREF1 and VREF2 are respectively connected to an internal resistor RREF and then connected to the same internal bias node. The operation of these two reference inputs is exactly the same. By connecting the corresponding voltage to the two reference voltage inputs, the output bias can be completed. In bipolar output mode, there are generally the following two connection methods.

1) When the input bidirectional current amplitude is the same, connect both reference voltage input terminals to the output terminal of an external reference voltage source, as shown in Figure 3(a). When the input voltage is negative relative to -IN, the output voltage will drop from the reference voltage. Conversely, when the input voltage is positive relative to -IN, the output voltage will rise from the reference voltage.

2) Connect one of the two reference voltage input terminals to the power supply voltage V+ terminal, and the other reference voltage input terminal to ground, as shown in Figure 3(b). When the input differential voltage is 0, the output voltage is biased to the middle of the power supply voltage of the AD8205. The advantage of this connection method is that no external reference source is required when measuring bidirectional current, and the output will automatically follow the change of the power supply voltage in a ratio to produce a half-amplitude bias. In other words, regardless of whether the power supply voltage rises or falls, the output bias point will always remain in the middle of the power supply voltage. For example, when the power supply voltage is 5V, the output is biased to 2.5V; and when the power supply voltage rises by 10%, the output will be biased to 2.75V.

Typical Applications

High-side current sensor and low-side switch method

As shown in Figure 4(a), the source of the PWM control switch is connected to the reference ground, and the inductive load and the sampling resistor are connected in series between the power supply and the PWM control switch. When the PWM switch is closed, the common-mode voltage on the sampling resistor drops to a value close to the negative peak value; when the PWM switch is opened, the common-mode voltage generated on the sampling resistor is the sum of the power supply voltage and the forward voltage drop of the freewheeling diode. The advantage of this method is that when the PWM switch is closed, since the sampling resistor is placed on the high side of the power supply, the sampling resistor is still in the current loop, so that the entire current on the load, including the freewheeling current, can still be monitored, and it is easy to identify the short-circuit fault to the ground to achieve short-circuit protection of the circuit.

High-side current sensor and high-side switch method

Connected as shown in Figure 4(b), the PWM switch and the sampling resistor are both on the high voltage side. When the PWM switch is turned on, the load power supply will be removed, but the freewheeling current can still be provided and monitored to achieve current control diagnosis. During operation, the power supply is isolated from the load most of the time, which can minimize the adverse effects caused by the differential voltage between the load and the ground. When the PWM switch is closed, the power supply voltage will be connected to the load, and the common-mode voltage will increase to the power supply voltage. When the PWM switch is turned on, the voltage will reverse and pass through the inductive load. Due to the effect of the freewheeling diode, the common-mode voltage on the sampling resistor is maintained at a diode conduction voltage drop below ground.

Motor Control

As shown in Figure 5, AD8205 is used as part of the control loop in the H-bridge motor control circuit. The motor and the sampling resistor are connected in series and placed in the middle of the H-bridge. By detecting the voltage on the sampling resistor, the current current and direction of the motor can be accurately measured. At this time, the output of AD8205 is set to an external reference bidirectional mode, so that it can measure the bidirectional current of the H-bridge switch and monitor the direction of the motor at the same time. Since the ground is not a particularly stable reference level, using the ground as a reference will lead to inaccurate measurements. Therefore, this test scheme is much better than the test method using the ground as the reference level.

Conclusion

In short, the circuit structure of using AD8205 to detect small differential voltage under high common mode voltage is simple and reliable, with high monitoring accuracy. It is particularly suitable for control systems such as power monitoring, hydraulic control, and magnetic suspension control of 42V automotive systems. ■