Cirrus Logic AN331 - Manual

1
Copyright © Cirrus Logic, Inc. 2008
(All Rights Reserved)
http://www.cirrus.com
AN331
Increasing ADC Dynamic Range with Channel Summation
by
Steve Green
1. Introduction
A commonly used technique to increase the system dynamic range of audio converters is to operate two converter
channels in parallel with the same signal and sum the outputs. The summation of the correlated signals creates a
6 dB increase in signal level while the summation of the uncorrelated noise sources increases the noise level by
only 3 dB. This summation effectively results in a 3 dB increase in dynamic range compared to each individual chan-
nel. This technique is most commonly associated with digital-to-analog converters but is also applicable to analog-
to-digital converters; as presented at the 87
th
AES Convention, “An 18-bit Dual Channel Oversampling Delta-Sigma
A/D Converter, with 19-bit Mono Application Example” by Clifton Sanchez of Crystal Semiconductor. In the case of
an A/D converter, it may be necessary to divide each of the digital signals by two prior to summation to avoid signal
overload in the processor. This approach is shown in the equations below, where A represents the signal in channel
A, B the signal in channel B and e
0
is the summed signal.
e
o
= A/2 + B/2
If A = B
e
o
= A/2 + A/2
e
o
= A
Another approach, which achieves the identical mathematical results, is to invert one of the analog inputs prior to
conversion and perform a subtraction of the two independent digital outputs. The advantage of this approach is that
any common in-phase signal between the individual digital output signals that may be introduced during the conver-
sion process (e
N
) is cancelled in the subtraction. This approach is shown in the equations below.
e
o
= (A + e
N
) / 2) - (B+ e
N
) / 2)
If B = - A
e
o
= ((A + e
N
) / 2)) - ((-A + e
N
) / 2)
e
o
= A
Though applicable to any A/D converter summing channels, using either technique, to increase dynamic range is
generally implemented in applications requiring the ultimate in dynamic range. As a result, this technique is generally
utilized with the highest performance A/D converters that are available. This application note will demonstrate an
implementation using the CS5381, which achieves 120 dB dynamic range for each individual channel in a standard
two-channel configuration, to achieve 123 dB dynamic range.
AUG ‘08
AN331REV1