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AN331

Increasing ADC Dynamic Range with Channel Summation

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

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

is the summed signal.

= A/2 + B/2

If A = B

= A/2 + A/2

= 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

) is cancelled in the subtraction. This approach is shown in the equations below.

= (A + e

) / 2) - (B+ e

) / 2)

If B = - A

= ((A + e

) / 2)) - ((-A + e

) / 2)

= 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

1 2 3 4 5 6 7 8

Inhaltsverzeichnis

Seite 1 - 1. Introduction

Seite 2

AN33122. Implementation Requirements for the CS5381The block diagram shown in Figure 1 shows an implementation of the CS5381 A/D. Notice that the same

Seite 3

AN33133. Recommended Analog Input Buffers for the CS5381 in Mono-ModeAn implementation with the CS5381 requires separate input buffer stages for the d

Seite 4 - 3.1 The Subtraction Approach

AN33143.1 The Subtraction ApproachThe schematic in Figure 3 is a suggested buffer implementation for the analog inversion with digital subtraction tec

Seite 5 - 4.1 The Subtraction Approach

AN33154. Demonstrating the TechniqueAssembling a test system to demonstrate this technique is a relatively simple matter using standard Cirrus Logicev

Seite 6 - Mono Mode

AN3316Figure 5. A-Weighted THD+N versus Amplitude at 1 kHz-130-100-128-126-124-122-120-118-116-114-112-110-108-106-104-102dBFS-120 +0-110 -100 -90 -

Seite 7 - 1 AUG 2008 Initial Release

AN3317Revision Date Changes1 AUG 2008 Initial Release

Seite 8

AN3318Contacting Cirrus Logic SupportFor all product questions and inquiries contact a Cirrus Logic Sales Representative. To find one nearest you go t

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