How to accurately measure current and voltage in a GSM system

In many wireless base station applications, power to the isolated power converter is provided through a -48 V supply. The voltage and the power supply current are normally monitored discretely with a couple of op amps and then fed to a dual channel ADC. They can also be monitored by using a traditional power monitor which integrates the current sensing/voltage measurement into a single device.

While this is a satisfactory solution in most cases, accuracy suffers when the base station is transmitting bursts of data and there is a drop in the voltage value and an increase in the current consumption due to the increased load. In these cases, accurate measurement requires sampling the voltage and the current at the exact point in time (with no delay due to multiplexing the voltage and current values) and a higher BW/faster analog digital converter (ADC) to capture the quickly varying signal. As an example, consider the GSM (Global System for Mobile communications) multiframe below, which consists of 26 frames of 8 time slots.

Each time slot includes a 96usec period called a training period, where well controlled data is transmitted so adaptive equalization is done (value is known in advance by receiver). Adaptive equalization overcomes fast changing multi-path fading and propagation effects. By accurately measuring the voltage and current during this 96usec window, the operator can determine if the transmitter is operating correctly.

The LMP92064 is well suited for this application. The device includes a current sense amplifier with input-referred offset of ±15uV and gain error of < ±0.75% to measure a load current across a shunt resistor and a buffered voltage channel with <2mV offset error and < ±0.75% of gain error to measure the supply voltage. The current and the voltage channels are sampled simultaneously by independent 125kSps 12-bit ADCs, allowing the designer to capture real time data at the GSM repeat rate with 12x oversampling.

The LMP92064 communicates through a 4-wire, 20MHz, SPI interface, allowing users to take advantage of the higher bandwidth ADC.

In summary, there are various ways to measure current and voltage, but a combination of fast conversion rate, simultaneous sampling and fast interface, like in the LMP92064, is quite effective in capturing the data accurately.