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Output impedance calculation

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Sep 12, 2009
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microcontroller output impedance

The DDR SDRAM controller in I.MX25 micro has the following characteristics as per its datasheet/reference manual.

“For DDRTYPE=00 (DDR2 or mDDR) at 25o C in the typical case, the following drive strengths can be referenced: Standard (3.6 mA), High (7.2 mA), Max (10.8 mA) with equivalent impedances of 90ohm, 45ohm, and 20ohmrespectively.”

It also has the logic levels of min 1.44v (0.8 * 1.8v VDD) for Voh (High) and max 0.36v (0.2* 1.8v VDD) for Vol (Low).

The selected drive strength, for example, is 3.6 mA at an impedance of 90ohm. If we apply ohm’s law with logic high voltage of 1.44v and impedance of 90 ohm the source current should be 16 mA but it is 3.6 mA as per the datasheet.

What is wrong with my calculation? The data sheets of most of the microcontrollers do not specify the output impedance at their digital I/O and peripheral pins. So I need to calculate the output impedance (Zo) for impedance matching the PCB traces. How do I know the output impedance (Zo) if it is not explicitly given in the datasheet.

how to calculate output impedance


There are several application note on the site of DRAM manufacture like "micron"

who will show you how to design the PCB tracks for the right impedance

Also Freescale have very good application note on who to design DRAM with I.MX processors

All the best


I think, the said application notes don't cover the estimation of source impedance from driver datasheets.

Basically, the source impedance must be calculated from the drivers differential impedance. In the said example it would be 0.36 V/ 3.6 mA = 100 ohm. But 0.36 is the maximum voltage, the typical voltage is somewhat lower, resulting in e.g. 90 ohm.

Because of the nonlinear output characteristics, you get different impedance values depending on the connected load. For a voltage referenced SSTL standard and impedance matching, 0.25 and 0.75*VDD are the nominal output voltages, very near to the specified output voltages.

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