ALLGEMEINE BESCHREIBUNG

The AD8317 is a demodulating logarithmic amplifier, capable of accurately converting an RF input signal to a corresponding decibel-scaled output. It employs the progressive compression technique over a cascaded amplifier chain, each stage of which is equipped with a detector cell. The device can be used in either measurement or controller modes. The AD8317 maintains accurate log conformance for signals of 1 MHz to 8 GHz and provides useful operation to 10 GHz. The input dynamic range is typically 55 dB (referenced to 50 Ω) with less than ±3 dB error. The AD8317 has 6 ns/10 ns response time (fall time/rise time) that enables RF burst detection to a pulse rate of beyond 50 MHz. The device provides unprecedented logarithmic intercept stability vs. ambient temperature conditions. A supply of 3.0 V to 5.5 V is required to power the device. Current consumption is typically 22 mA, and it decreases to 200 μA when the device is disabled.  The AD8317 can be configured to provide a control voltage to a power amplifier or a measurement output from the VOUT pin. Because the output can be used for controller applications, special attention has been paid to minimize wideband noise. In this mode, the setpoint control voltage is applied to the VSET pin. The feedback loop through an RF amplifier is closed via VOUT, the output of which regulates the output of the amplifier to a magnitude corresponding to VSET. The AD8317 provides 0 V to (VPOS − 0.1 V) output capability at the VOUT pin, suitable for controller applications. As a measurement device, VOUT is externally connected to VSET to produce an output voltage, VOUT, that is a decreasing linear-in-dB function of the RF input signal amplitude. The logarithmic slope is -22 mV/dB, determined by the VSET interface. The intercept is 15 dBm (referenced to 50 Ω, CW input) using the INHI input. These parameters are very stable against supply and temperature variations.  The AD8317 is fabricated on a SiGe bipolar IC process and is available in a 2 mm × 3 mm, 8-lead LFCSP with an operating temperature range of −40°C to +85°C.

 

FEATURES

Wide bandwidth: 1 MHz to 10 GHz

High accuracy: ±1.0 dB over temperature

55 dB dynamic range up to 8 GHz ± 3 dB error

Stability over temperature: ±0.5 dB

Low noise measurement/controller output, VOUT

Pulse response time: 6 ns/10 ns (fall/rise)

Small footprint, 2 mm × 3 mm LFCSP

Supply operation: 3.0 V to 5.5 V at 22 mA

Fabricated using high speed SiGe process

 

ANWENDUNGEN

RF transmitter PA setpoint control and level monitoring

Power monitoring in radio link transmitters

RSSI measurement in base stations, WLANs, WiMAX, and radars

 

ARBEITSTHEORIE

The AD8317 is a 6-stage demodulating logarithmic amplifier, specifically designed for use in RF measurement and power control applications at frequencies up to 10 GHz. Sharing much of its design with the AD8318 logarithmic detector/controller, the AD8317 maintains tight intercept variability vs. temperature over a 50 dB range. Additional enhancements over the AD8318, such as a reduced RF burst response time of 6 ns to 10 ns, 22 mA supply current, and board space requirements of only 2 mm × 3 mm, add to the low cost and high performance benefits of the AD8317. A fully differential design, using a proprietary, high speed SiGe process, extends high frequency performance. Input INHI receives the signal with a low frequency impedance of nominally 500 Ω in parallel with 0.7 pF. The maximum input with ±1 dB logconformance error is typically 0 dBm (referenced to 50 Ω). The noise spectral density referred to the input is 1.15 nV/ÖHz, which is equivalent to a voltage of 118 μV rms in a 10.5 GHz bandwidth or a noise power of −66 dBm (referenced to 50 Ω). This noise spectral density sets the lower limit of the dynamic range. However, the low end accuracy of the AD8317 is enhanced by specially shaping the demodulating transfer  characteristic to partially compensate for errors due to internal noise. The common pin, COMM, provides a quality low impedance connection to the printed circuit board (PCB) ground. The package paddle, which is internally connected to the COMM pin, must also be grounded to the PCB to reduce thermal impedance from the die to the PCB.