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
Above: This signal at DC level shows clearly the maximum output voltage of .72 Volt. Ro = 10kOhm (so max clipping)
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:
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.
Doede Douma, December 31, 2000