Silicon Chip – May 2019

32 Silicon chip Australia’s electronics magazine siliconchip.com.au

The reason for DC biasing the two differential inputs dif-
ferently is to overcome a potential problem with analog-
to-digital converters, that when the signal is near the ‘zero
point’, the binary values at the output tend to flip between
all zeros and all ones. This can cause digital noise at the
worst possible time – when there is near silence at the inputs.
By adding a slight DC offset, the zero point is moved such
that any small amount of noise will only cause a few bits
to flip. That offset is removed by digital filtering inside the
ADC chip. While modern delta-sigma ADCs do not suffer
from this problem anywhere near as severely as early ADCs,
this solution is cheap insurance to guarantee that the bit
flipping problem does not affect us.
The bottom end of the divider which produces the half-
supply bias rails is bypassed with 10μF and 100nF capaci-
tors, to reject any noise and ripple that may be on this rail
and prevent it from getting into the signal path. The ADC
runs from its own regulated 5V rail which should be pret-

ty ‘quiet’. But this is a very high-performance ADC, so it

isn’t worth taking any risks in feeding noise into its inputs.

The 91series resistors at the op amp outputs protect

the ADC from excessive voltages. The op amps run from

±9V while the ADC runs from 5V, so the op amps outputs

can swing beyond both of the ADC supply rails. But since

the op amp feedback comes from after this resistor (ie, it’s

inside their feedback loops), the output impedance is still

very low, and the frequency response is flat.

Schottky diodes D5, D6, D9 & D10 help to further protect

the ADC inputs, by conducting if the op amps try to drive

the ADC inputs below -0.3V or above +5.3V. This prevents

any standard silicon devices (eg, transistors or diodes) in-

side IC1 from conducting due to an excessive input voltage,

as usually this will only happen once the applied voltage

is more than 0.6V beyond the supply rails.

The 91resistors also combine with a 2.7nF capacitor

across the differential inputs of IC1, to provide some further

Fig.5: the DAC board does the opposite of the ADC board, converting the digital

audio signals from the microcontroller back to balanced analog signals, then

converting these to single-ended audio signals so they can be fed to stereo RCA

output connector CON4.