High-precision true RMS power detector AD8361 with frequency response up to 2.5GHz

    Abstract: AD8361 is a high-precision true RMS power detector produced by AD Company. It uses a unique double-square unit closed-loop ratio-efficiency conversion circuit technology, so it has excellent performance. It can be used to test the radio frequency transmitting and receiving signal chain in the 2.5GHz high frequency range and to measure CDMA, W-CDMA, QAM and other complex modulation waveforms. The article gives detailed technical parameters and various typical performance analyses.

    Keywords: effective value power square AD8361

1 Overview

    AD8361 is a true effective value power detector that can be used to test transmitting and receiving signal chains in a high frequency range up to 2.5GHz. It uses a single power supply and can work between 2.7V and 5.5V. In most applications, only a single supply decoupling capacitor and input coupling capacitor will work and the output will respond linearly (the DC output has a conversion gain of 7.5V/Vrms). Adding an external filter capacitor can increase the averaging time constant.

    AD8361 is particularly suitable for true RMS power measurement of various simple or complex waveforms, especially for measuring signals with high peak modulation coefficients (peak-to-RMS ratio), such as CDMA and W-CD-MA signals.

    In order to meet the requirements of various analog/digital converters, AD8361 has the following three working modes:

    ●Ground reference mode, the initial value is 0;

    ●Internal reference mode, the initial value output is 350mV higher than the ground level;

    ●Power reference mode, the initial value is Vs/7.5 (Vs is the power supply voltage).

    ●AD8361 can operate in the temperature range of -40℃~+85℃, adopts 8-pin micro SOIC package (single integrated circuit), uses special high load-stop frequency fT silicon wafer and is manufactured using bipolar process. Therefore, it has the following characteristics:

    ●Corrected true RMS response;

    ●Excellent temperature stability;

    ●The input range at 2.5GHz is up to 30dB;

    ●Maximum input 700mV rms, 30dB, 50Ω resistance;

    ●When the frequency is as high as 2.5GHz, there is still a linear response of ±0.25dB;

    ●Single power supply operation: 2.7V～5.5V;

    ●Low power consumption: 3.3mW when powered by 3V;

    ●Quickly reduce power consumption, current is less than 1μA;

    ●Used for measurement of CDMA, W-CDMA, QAM and other complex modulation waveforms.

2 Pin functions and parameters

2.1 Pin function

    The pin arrangement of AD8361 is shown in Figure 1. The functions of each pin are as follows:

    ●VPOS: power supply voltage input terminal. The working range is 2.7V~5.5V.

    ●IREF: Output reference control terminal. This terminal is left floating to enable the internal reference working mode; in other working modes, this terminal should be connected to the VPOS terminal. This terminal cannot be connected to ground.

    ●RFIN: signal input terminal. This end must be driven by a coupled power supply. At low frequencies, the actual input impedance is 225Ω.

    ●PWDN: Reduce power consumption control terminal. When this terminal is connected to a logic low level (less than 100mV), the device will operate normally; when it is connected to a logic high level (greater than Vs-0.5V), the device will be shut down and the supply current will tend to 0 (grounded and internal reference mode) The current is less than 1μA; in power reference mode, the power supply is blocked by a 100kΩ resistor).

    ●COMM: Common ground terminal of the device.

    ●FLTR: Configuring a capacitor between this end and the VPOS end can reduce the turning point frequency of the modulation filter. For small signal input, the on-chip 27pF capacitor and 2kΩ resistor are connected in parallel to form a filter.

    ●VRMS: output voltage terminal. The output has limited drive and near rail-to-rail voltage capabilities. The load resistance of the subsequent stage should be greater than 10kΩ.

    ●SREF: Power reference control terminal. Connect this terminal to VPOS to enable the power supply reference working mode; in other working modes, SREF is connected to the COMM terminal (grounded).

2.2 Limit parameters

    The following are the extreme operating parameters of AD8361:

    ●Power supply voltage Vs:5.5V;

    ●SREF and PWDN terminal inputs are 0V and Vs respectively;

    ●IREF terminal input: Vs-0.3V, Vs;

    ●RFIN terminal input: 1V rms;

    When the equivalent power resistance is 50Ω, it is 3dBm;

    ●Internal power dissipation: 200mW;

    ●Junction temperature thermal resistance QJA: 200℃/W;

    ●Maximum junction temperature: 125℃;

    ●Operating temperature range: -40℃～+85℃;

    ●Storage temperature range: -65℃～+150℃;

    ●Pin withstand temperature range (soldering time within 60 seconds): 300℃.

3 Working principle

    The AD8361 is an RMS response detector. It provides an accurate method of measuring RF power that is essentially waveform independent (true RMS). Its internal circuit block diagram is shown in Figure 2. The AD8361 device uses a unique circuit technology that has two identical square cells and uses a high-gain error amplifier to balance the outputs of the two square cells.

    If the signal under test is connected to the input terminal of the first square unit, the input impedance between the RFIN terminal and the COMM terminal is 225Ω. Since the input pin has a voltage of about 0.8V relative to the ground level, a coupling capacitor must be connected. Thanks to this external component, the measurement range can be extended to arbitrarily low frequency ranges.

    When the input voltage value at the AD8361 input terminal is VIN, the squaring unit will generate a current proportional to the square of the voltage VIN. This current forms a low-pass filter through the internal load resistor and capacitor, and the average value of the square of the voltage VIN can be extracted. , although the voltage response is essentially related to the input impedance, it can be corrected according to the equivalent power so that the input voltage corresponding to 1mW is 4477mV rms. In the application example section of this article, we will explain how to achieve input and 50Ω impedance matching.

    The amplitude of the input voltage after passing through the low-pass filter may be reduced, which can be added to one end of the error amplifier. The second identical voltage square unit forms a closed negative feedback circuit for this error amplifier. The second square unit is driven by the divided voltage of the quasi-DC output voltage of AD8361. When the input voltage of the second square unit is equal to VIN When the effective value is reached, the closed-loop circuit is in a stable state. At this time, the output voltage is the effective value of the input voltage. The feedback coefficient of this feedback loop is 0.133, and the conversion gain of rms-dc is 7.5, that is:

    V out =7.5V IN rms

    The feedback channel formed by the second square unit can be used to achieve precise measurement of the received signal. This method has two advantages: first, the influence of the range change is eliminated in the two square units; second, the temperature rise When , the response tracking processes of the two square units are very close, resulting in excellent calibration stability.

    Square elements have a very wide bandwidth and thus an inherent response from DC to microwave. However, a small error signal will be generated at the bottom end of the dynamic range, and due to a small internal offset, its accuracy will be affected somewhat at small signals.

    On the other hand, the square cell in the AD8361 has characteristics of the "AB" class: the input peak is not limited by the static bias condition, but mainly depends on the eventual loss of square-law consistency. The result is that the top end of the squared cell's response range occurs at considerable input voltages (approximately 700mV rms), and in practice the maximum usable range will be limited by the output swing. The rail-to-rail output can swing from a few millivolts above ground to a voltage just below the supply voltage (less than 100mV).

4 Applications of AD8361

4.1 Basic connection method

    Figure 3 shows the basic connection method of AD8361 using power reference. The device uses a single power supply (between 2.7V and 5.5V). The VPOS terminal is decoupled by 100pF and 0.01μF capacitors. In the working state, its quiescent current is 1.1mA. PWDN can be connected to VPOS to reduce the quiescent current. to 1μA.

    Connecting an external 75Ω resistor to the AC-coupled input equates to an input impedance of approximately 50Ω covering the full bandwidth. Note: The coupling capacitor must be connected between the input terminal and the impedance of this branch.

    The high-pass corner frequency generated by the combination of the input coupling capacitor and the internal input resistor can be calculated by the following formula:

    f 3dB =1/2πC c R in

    The circuit in Figure 3 uses a 10pF capacitor, and its high-pass turning frequency is about 8MHz.

    The nominal value of the output voltage is 7.5 times the effective value of the input voltage (7.5V/V rms conversion gain). Three different working modes, ground reference, internal reference and power reference, can be set by the SREF and IREF terminals. In the ground-referenced mode shown in Figure 3, the output voltage can swing from near ground to 4.9V on a 5V supply. If the IREF and VPOS terminals are connected, the internal reference mode is set. The connection method of the power reference method is to connect the SREF terminal and the VPOS terminal based on the internal reference method. In these two modes, a compensation voltage is allowed to be added to the output terminal. In the internal reference mode, the output voltage stretcher amplitude will be shifted upward by the internal reference voltage of 350mV; in the power reference mode, the compensation voltage of Vs/7.5 is added to the output voltage. Table 1 summarizes the connections, output conversion factors, and output voltages in the three modes.

Table 1 Relationship between three connection methods and nominal conversion

    The AD8361 chip can produce an output current of nearly 3mA, and using an operational amplifier driver circuit at the output can increase or decrease the nominal output of 7.5V/V rms (see Figure 3). A slope of 3.75V/V rms can be obtained by dividing the voltage with the potentiometer. Adjusting the gain of the op amp can also increase or decrease the slope. If high output current (>10mA) is required, the rail-to-rail capability of the AD8361 can be used, which allows currents up to 45mA.