PCM 63 I/V resistor value tests
All signals at unloaded Iout from PCM63

The PCM 63 has protecting diodes in the Output circuits. This could result in heavy distortion / (soft) clipping when the signal goes over the conducting point of the diodes. The maximum current from the PCM 63 is 4mA peak-peak. So the value of the resistor (Ro) should be selected in such a way that the Vout does not exceed this voltage (round 0.7Volt) at max I out. Below are the results of a test done with a digital scope and Spectrum analyzer. The input signal was a test tone CD with 1kHz at 0dB

MAXVOUT.jpg (16124 bytes)

Above:    This signal at DC level shows clearly the maximum output voltage of .72 Volt. Ro = 10kOhm (so max clipping)


DISMAX.jpg (14344 bytes) DISFMAX.jpg (15457 bytes)

Above:  Left picture shows the output signal with at the right the resulting Spectrum. k2 at -36dB is the absolute maximum acceptable level of distortion. Vout p-p is 0.63 Volt, resulting in Ro of 160 Ohm.

Already at 200 Ohm (!!!!!) the level of distortion is already unacceptable.
See picture below:

2OHM.jpg (15450 bytes) 2FOHM.jpg (15977 bytes)


The best solution is found at 100 Ohm. See picture below. Here we have a good compromise between low distortion (-55dB), high headroom and Output voltage of 130mV effective.

100OHM.jpg (14732 bytes) 100FOHM.jpg (14944 bytes)

Doede Douma, December 31, 2000