This circuit uses the ADL5902 TruPwr™ detector to measure the rms signal strength of an RF signal with a signal crest factor (peak-to-average ratio) that varies over a dynamic range of approximately 65 dB, operating from 50 MHz to 9 GHz.

The measurement results are provided as serial data at the output of the 12-bit ADC ( AD7466 ). Perform a simple 4-point system calibration in the digital domain for ambient temperature.

The interface between the RF detector and the ADC is simple, consisting of two signal conditioning resistors and no active components. In addition, the ADL5902 internal 2.3 V reference voltage provides power and reference voltage for micropower ADCs. The AD7466 has no pipeline delay and can be used as a read-only SAR ADC.

The entire circuit achieves approximately ±0.5 dB temperature stability.

Data shown is for two devices operating over the −40°C to +85°C temperature range.

The measured RF signal is applied to the input of the ADL5902, a dB linear rms response root mean square detector. The external 60.4 Ω resistor R3, combined with the higher input impedance of the ADL5902, ensures a wideband 50 Ω match to the RF input. The ADL5902 is configured in so-called "measurement mode" with the VSET and VOUT pins connected. In this mode, the output voltage is proportional to the logarithm of the input rms value. In other words, the reading is presented directly in decibels, adjusted to 1.06 V per decade, or 53 mV/dB.

The supply voltage and reference voltage for the AD7466 12-bit ADC are provided by the ADL5902 internal 2.3 V reference voltage source. Because the AD7466 consumes so little current (only 16 μA when sampling at 10 kSPS), the ADL5902's reference voltage output is sufficient to power the ADC and the temperature compensation and rms accuracy adjustment network consisting of R9, R10, R11, and R12.

The ADC full-scale voltage is equal to 2.3 V. The maximum detector output voltage (when operating in the linear input range) is approximately 3.5 V (see ADL5902 data sheet Figures 6, 7, 8, 12, 13, and 14) and therefore must be reduced by a factor of 0.657 before driving the AD7466. This reduction is accomplished with simple resistor dividers R10 and R11 (1.21 kΩ and 2.0 kΩ). The above values ​​achieve a practical scaling factor of 0.623, which ensures that the ADL5902 RF detector does not overdrive the ADC by establishing resistor tolerance margin.

Figure 2 shows a typical curve of detector output voltage versus input power (without output adjustment).

The transfer function of this detector can be approximated by:

VOUT = SLOPE_DETECTOR × (PIN − INTERCEPT)

Where SLOPE_DETECTOR is the detector slope, in mV/dB; INTERCEPT is the x-axis intercept, in dBm; PIN is the input power, in dBm.

At the ADC output, VOUT is replaced by the ADC output code, and the formula can be rewritten as:

CODE = SLOPE × (PIN − INTERCEPT)

Among them, SLOPE is the combined slope of the detector, adjustment resistor and ADC, and the unit is times/dB; the unit of PIN and INTERCEPT is still dBm.

Figure 3 shows a power sweep of a typical detector input power and the ADC output code observed with a 700 MHz input signal.

The overall slope and intercept vary from system to system due to device-to-device differences in the RF detector, trim resistor, and ADC transfer function. System-level calibration is therefore required to determine the slope and intercept of the entire system. In this application, a 4-point calibration is used to correct for some nonlinearity within the RF detector transfer function, especially at the low end. This 4-point calibration scheme produces three slope and three intercept calibration coefficients, and these values ​​should be stored in non-volatile RAM (NVM) after calibration.

Calibration is performed by applying four known signal levels to the ADL5902 and the corresponding output codes are measured from the ADC. The selected calibration point should be within the linear operating range of the device. In this example, the calibration points are at 0 dBm, −20 dBm, −45 dBm, and −58 dBm.

The slope and intercept calibration coefficients are calculated using the following formula:

SLOPE1 = (CODE_1 – CODE_2)/(PIN_1 − PIN_2)

INTERCEPT1= CODE_1/(SLOPE_ADC × PIN_1)

Then repeat the calculation using CODE_2/CODE_3 and CODE_3/CODE_4 to obtain SLOPE2/INTERCEPT2 and SLOPE3/INTERCEPT3 respectively. The six calibration coefficients should be stored in the NVM along with CODE_1, CODE_2, CODE_3, and CODE_4.

When the circuit is operating in the field, these calibration coefficients are used to calculate the unknown input power level PIN as follows:

PIN = (CODE/SLOPE) + INTERCEPT

In order to obtain the appropriate slope and intercept calibration coefficients during operation of the circuit, the CODE observed from the ADC must be compared with CODE_1, CODE_2, CODE_3, CODE_4. For example, if the CODE from the ADC is between CODE_1 and CODE_2, SLOPE1 and INTERCEPT1 should be used. This step can also be used to provide underrange or overrange warnings. For example, if the CODE from the ADC is greater than CODE_1 or less than CODE_4, it means that the measured power is outside the calibration range.

Figure 3 also shows the relationship between the change of the circuit transfer function and the above straight-line formula. This error function is caused by edge curvature of the transfer function, small ripple in the linear operating range, and temperature drift. The error is expressed in dB and the formula is as follows:

Error (dB) = calculated RF power − actual input power
= (CODE/SLOPE) + INTERCEPT – PIN_TRUE

Figure 3 also includes a plot of error versus temperature. In this example, the ADC codes measured at +85°C and −40°C are compared to the straight-line formula at ambient temperature. This approach is consistent with real-world systems, where system calibration can generally only be performed at ambient temperature.

Figures 4 and 5 show the circuit performance at 1 GHz and 2.2 GHz respectively.

The performance of this or any high-speed circuit is highly dependent on proper PCB layout, including but not limited to power supply bypassing, controlled impedance lines (if required), component placement, signal routing, and power and ground planes. (For details on PCB layout, see the MT-031 Tutorial , the MT-101 Tutorial , and the Practical Guide to High-Speed ​​Printed Circuit Board Layout articles.)