Circuit functions and advantages
− 48 V power rails are widely used in wireless base stations and telecommunications equipment. When used in the network central switching office, it can vary from − 48 V to − 60 v. When measuring the current at this voltage, devices powered by dual power supplies (e.g. ± 15 V) are usually required. Generally speaking, only the front-end conditioning amplifier directly connected to the − 48 V supply rail uses dual power supply, and the rest of the system uses single power supply. However, removing the negative power supply can simplify the circuit and reduce the cost. This circuit uses AD629 and ad8603. It only uses positive power supply, but it can also measure the current from − 48 V to − 60 v.
Compared with low-end current detection, high-end current detection can suppress grounding noise and detect short-circuit conditions during operation.
Figure 1: circuit for measuring − 48 V current (schematic diagram)
This circuit uses differential amplifier AD629 to regulate the voltage beyond its power supply. The minimum and maximum allowable input common mode voltages are determined by the following formula:
VCOM_ MAX = 20 &TImes; （+VS – 1.2） – 19 &TImes; VREF
VCOM_ MIN = 20 &TImes; （−VS + 1.2） – 19 &TImes; VREF
When VREF = + 5 V, + vs = 12 V and − vs = 0 V, the AD629 common mode input range is − 71 V to + 121 V, which is sufficient to cover the entire expected range of − 48 V supply rail. Differential amplifier AD629 detects differential voltage is × RS, which is generated by the current flowing through the shunt resistor. The AD629 has a fixed gain of 1, so its output voltage is equal to is × RS +VREF。
Shunt resistance is 100 m Ω, tolerance is 0.1%, and the maximum rated power is 1 W. When selecting shunt resistance, current measurement accuracy and self heating effect should be considered.
The ad8603 is configured as a subtracter, so it can suppress the 5 V common mode voltage and amplify the target signal is × RS。 The signal is amplified 20 times to match the 2.5 V full-scale input range of the ad7453 ADC. The full-scale 2.5 V input signal of the ADC corresponds to 1.25 a of the − 48 V power supply. The ad8603 was selected because of its low input bias current, low offset drift, and rail to rail input and output characteristics. Rail to rail output enables the ad8603 to share the same power supply with the ADC. It should be noted that due to the existence of the output stage, the output of the ad8603 can only be reduced to about 50 MV above ground, and the corresponding input current is is about 25 mA. Therefore, this circuit cannot measure a current below approximately 25 mA. However, it is not usually required to measure very low currents with high accuracy.
The ratios of the four resistors that make up the subtractor must be matched to obtain maximum common mode rejection (CMR) performance. In this stage, the subtracter must suppress the 5 V common mode signal of the AD629.
The reason for using 12 bit ADC ad7453 is that it has Pseudo differential input, which can simplify the interface between ad8603 and ADC. In addition, the ADC is packaged in small size and low cost, so it is suitable for cost sensitive or size limited applications.
The ad780 has high precision and is easy to use, so it is selected as the reference voltage source for the 12 bit ADC ad7453.
We have tested this circuit for − 48 V and − 60 V supply rails, and the measured digital output voltage as a function of current is shown in Figure 2. It can be seen from the figure that the actual value is highly correlated with the expected value, and the circuit has good linearity under different common mode voltages.
Figure 2: relationship between digital output voltage and current at − 48 V and − 60 V common mode voltage