Analog Devices ee-26 Application Notes

Engineer to Engineer Note EE-26

Notes on using Analog Devices’ DSP, audio, & video components from the Computer Products Division

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AD184x Sigma Delta Converters:
How well do they work with DC
inputs?

Last Modified: 8/19/97

Overview

This Engineer’s Note will provide some insight into the
AD18xx Sigma Delta Converters and Codecs to be used
for DC inputs. The AD184x parts are normally AC­coupled, since they were designed specifically for audio
applications which sample sinusoidal signals and reject
DC signals.

1. What do you mean by AC-coupling?

The AD1849, AD1847 and AD1843 all are accept
Industry-Standard compact disk “line-level” inputs. Line­level inputs are 2 V RMS centered around analog ground..
( For other audio equipment, “line-level” is more loosely
defined.) The AD184x codecs are +5 V analog supply
powered devices. Line level voltage swings for these parts
are defined to be 1 V rms for and ADC input and 0.707 V
rms for the DAC output. Thus, 2 V RMS input analog
signals must be attenuated and either centered around the
reference voltage intermediate between 0 V and +5 V or
AC-coupled. The AD1847’s VREF pin ( or CMOUT for
the 1843 and 1849K) will be at this intermediate voltage,
nominally 2.25 volts. It has limited drive but can be used
as a voltage datum to an op amp input. Note, however,
that DC-coupled input are not recommended, as they
provide no performance benefits with the AD184x
architecture. Furthermore, DC offset differences between
multiple DC-coupled inputs create the potential for
“clicks” when changing the input mux selection.

intend biasing the driving op-amp with the
AD1849k CMOUT, so that the only difference in
DC levels will be that caused by the op-amp's
output offset.

Regarding AC vs. DC input signal coupling. Yes, if
you switch between different DC bias voltage levels, you
will generate switching noise. The input to the CODEC
internal Op-Amps, is biased to the CMOUT voltage, +/­the input offset of the Op-Amps. This input offset is
typically less than 5mV and will not cause serious
problems, i.e. 'pops & clicks' unless gained-up in
subsequent signal processing. If you think your system
may generate total input offsets in excess of 10 to 20 mV
you may want to determine, by experiment, the maximum
tolerable level of switched noise you can tolerate. Wider
bandwidth applications where high S/N ratios are expected
will be subjectively worse, especially if augmented by
ringing in any filters.

3. We want to work to below 1Hz (say 3 dB at

0.5 Hz) implies need 33uF or greater I/P
coupling caps, and our present AD1847 circuit
could give +ve or -ve bias across caps (i.e. dc
offset into I/P).

Is there a problem with dc coupling other than
that any offset would get amplified by the gain
block of the AD1847? We could have up to
100mV offset (so couldn't use much gain).

This part was designed for audio applications and a small
DC offset introduces 'pops-n-clicks' as the different inputs
are selected. Also the DC offset generates 'zipper' noise
when the gains are changed hence the reason for
discouraging DC input signals. We do have a suggested
circuit for use in DC applications you might want to see.
Email a request with your FAX number and we'll send
you the schematic.

2. Crystal data for the CS4215 says that the line
inputs should be DC coupled for minimum
output offset. The AD datasheets says there is no
advantage to DC coupling. Will DC coupling to
the AD1849k generate loud pops when switching
between the AC coupled mic input and the DC
coupled line input as the data sheet suggests? I
would have expected no noise as the inputs in
both cases would be at the same DC potential. I