Current-source DAC with PIN diode provides RF attenuation and thermal compensation

PIN diodes are often used as variable attenuators for RF signals in TV tuners, and for wideband RF in fixed equipment. These diodes can be mounted as discrete devices on a board, or integrated into hybrid GaAs modules. At high RF frequency, the forward resistance of a PIN diode decreases with increasing dc current through the junction (Figure 1 ).

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Figure 1: Resistance vs. forward current for a typical PIN diode; top image is BAR63J from ST Micro, bottom image is BAP50-05 from Philips.

PIN-diode attenuators can operate in a series or shunt configuration. A series attenuator, Figure 2a , typically requires 10 mA to 20 mA through the diode.

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Figure 2. An RF signal can be attenuated by a series (a) or shunt (b) PIN diode.

Its attenuation is:

Atten (in dB) = 20 log (1 + RPIN /2Z0 )

For a shunt attenuator, Figure 2b , the required bias current is typically 2 mA to 3 mA. The attenuation for a shunt attenuator is:

Atten (in dB) = 20 log (1 + Z0 /2RPIN )

A system controller adjusts the attenuation by varying current through the diode.

The system may also require temperature compensation, Figure 3 .

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Figure 3. A simplified bias circuit for the PIN diode.

In this simplified circuit, a digital potentiometer IC (digiPOT) sets the fixed or adjustable bias level, and a thermistor provides the temperature input. These inputs are combined in an op-amp circuit, and the output converted to current by driving the PIN diode through a resistor.

This implementation is not straightforward. The thermistor response must be matched to the PIN diode, and bias-current changes can alter the PIN diode’s DC forward voltage, which in turn causes nonlinearity in the bias current. You could replace the thermistor and digiPOT with a voltage-output digital/analog converter (DAC) and compensate the circuit digitally, but that approach would not eliminate the diode’s forward voltage. A much better alternative is to introduce a current-source DAC, Figure 4 .

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Figure 4: This RF attenuator is driven by a current-output DAC (MAX5548 or MAX5550), which also compensates for temperature by adjusting its output current according to correction signals from the host processor.

This circuit includes a hybrid quadrature matched-shunt attenuator and a pair of matched PIN diodes, which are driven by a single DAC output. Ambient temperature is measured by an analog or digital sensor connected to the host microcontroller, which uses a lookup table (LUT) or algorithm to implement the temperature compensation. The desired attenuation passes through the LUT or algorithm to give the PIN current needed, which is then set by the DAC. Current through the PIN-diode junction is set only by the DAC, and is independent of the diode’s forward voltage or any other dc impedance in the connection.

Filter on output
For circuits that include RF blocking inductors and capacitors (to eliminate unwanted dc paths), the outputs remain stable with more than 100 nH of series inductance and 10 nF of capacitance to ground. As one benefit of a current output (compared to a voltage output), series resistance introduced by the filter has no effect on accuracy, provided the resulting voltage drop remains within the DAC’s range of output-compliance voltage.

DAC description
The MAX5548 and MAX5550 DACs are dual 8- and 10-bit devices with current-source outputs and a compliance range of 4 V at 30 mA. Both outputs can source up to 30 mA, and they can be paralleled for high-current applications up to 60mA. Output-leakage current in shutdown mode is only ±1 μA maximum. To ensure high accuracy and low-noise performance, each IC includes a +1.25V bandgap reference and a control amplifier. They also offer the option of connecting an external reference for improved gain accuracy. Software and an external resistor for each output determine the maximum value for programmable output current.

About the authors
The late Terry Millward worked in the electronics industry for 24 years. In addition to service with British Aerospace, Crosfield Electronics, and Harris Semiconductor, he worked with Maxim Integrated Products Inc., Sunnyvale, CA for the last 13 years, most recently as the Field Applications Director for the Signal Processing and Conversion Products Business Unit.

Dave Devries is an RF engineer with 25 years industry experience, working in Maxim’s RF engineering group. He can be reached at (408) 737 7600.

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