Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part II - Design of High-Performance AVRs and Pre/Pros


Differential and Current Mode DACs

In this section, block diagrams for the analog electronics after the DACs are shown. The first diagram is for a balanced voltage mode DAC which has two outputs that are in anti-phase to each other.

The figure above shows how the output of differential voltage mode DAC is converted to a single ended signal using a balanced to single ended converter.

Distortion is reduced for two reasons:

  1. Subtracting these signals yields a single ended signal of twice the magnitude providing increased dynamic range.
  2. Even order distortion components in the individual signal, if correlated, will be canceled when the signals are subtracted.

The balanced-to-single-ended converter block is the same as the one shown in a previous figure of a balanced volume control.

In addition to the op-amp and resistors, additional reactive components may be added to form the Low Pass Filter (LPF) for the removal of high frequency, out off band energy, which occurs in the digital sampling process. More on this subject is discussed below.

Another method to reduce distortion at the output of the DAC is to transmit the signal in the current domain as shown in the figure below. In the chart from the previous part of this article, it can be seen the best current-mode DACs are superior to voltage-mode DACs for minimizing distortion.

In the current domain, the signal out of the DAC does not move in amplitude.

The green circuit block between the DAC and the balanced-to-single-ended converter converts the current flowing at the DACs output pins to a voltage. This is a current-to-voltage converter, often abbreviated as I/V converter. The circuit consists of an op-amp and a resistor in the feedback loop

Internal op-amps are absent in the typical current-mode DAC; instead, only switched current sources are present because it is difficult to integrate a high performing op-amp in the CMOS process technology.

Moving the op-amp outside the DAC permits an IC process technology designed for analog circuits to be used. The analog process technology provides improved noise performance and reduced distortion. The ability of the analog process to safely operate with a 30V power supply, rather than the 5 volts of power for the DAC, improves the signal-to-noise ratio.

Additional reactive components may be found as part of the I/V stage. These components create a low-pass filter for removal of out-of-band energy. This is typically a first-order (6 dB/octave) filter. The balanced-to-single-ended converter can provide a second-order (12 dB/octave) filtering function. By placing the poles of both filters optimally, a third-order (18 dB/octave) filter Butterworth filter is in the signal path.