reSIDfp::Dac Class Reference

#include <Dac.h>

Public Member Functions
	Dac (unsigned int bits)
void	kinkedDac (ChipModel chipModel)
double	getOutput (unsigned int input) const

Detailed Description

Estimate DAC nonlinearity. The SID DACs are built up as R-2R ladder as follows:

    n  n-1      2   1   0    VGND
    |   |       |   |   |      |   Termination
   2R  2R      2R  2R  2R     2R   only for
    |   |       |   |   |      |   MOS 8580
Vo  --R---R--...--R---R--    ---

All MOS 6581 DACs are missing a termination resistor at bit 0. This causes pronounced errors for the lower 4 - 5 bits (e.g. the output for bit 0 is actually equal to the output for bit 1), resulting in DAC discontinuities for the lower bits. In addition to this, the 6581 DACs exhibit further severe discontinuities for higher bits, which may be explained by a less than perfect match between the R and 2R resistors, or by output impedance in the NMOS transistors providing the bit voltages. A good approximation of the actual DAC output is achieved for 2R/R ~ 2.20.

The MOS 8580 DACs, on the other hand, do not exhibit any discontinuities. These DACs include the correct termination resistor, and also seem to have very accurately matched R and 2R resistors (2R/R = 2.00).

On the 6581 the output of the waveform and envelope DACs go through a voltage follower built with two NMOS:

        Vdd

         |
       |-+

Vin ----—| T1 (enhancement-mode) |-+ | o-----— Vout | |-+ +—| T2 (depletion-mode) | |-+ | |

GND GND

Constructor & Destructor Documentation

Dac()

reSIDfp::Dac::Dac

(

unsigned int

bits

)

Initialize DAC model.

Parameters

bits	the number of input bits

Member Function Documentation

getOutput()

double reSIDfp::Dac::getOutput

(

unsigned int

input

)

const

Get the Vo output for a given combination of input bits.

Parameters

input

the digital input

Returns

the analog output value

kinkedDac()

void reSIDfp::Dac::kinkedDac

(

ChipModel

chipModel

)

Build DAC model for specific chip.

Parameters

chipModel

6581 or 8580

---

The documentation for this class was generated from the following files:

• Dac.h
• Dac.cpp