EVAL-AD1871EBZ Datasheet, PDF (25/28 Pages) Analog Devices – Stereo Audio, 24-Bit, 96 kHz, Multibit
- ADC

INTERFACING
Analog Interfacing
The analog section of the AD1871 has been designed to offer
flexibility as well as high performance. Users may choose full
differential input directly to the ADC’s ⌺-⌬ modulator via Pins
CAPxP and CAPxN. Alternatively, when using the on-chip PGA
section, it is also possible to multiplex single-ended inputs on Pins
VINxP and VINxN or to use these pins for full differential input.
Whichever input topology is chosen (direct or via mux/PGA
section), the modulator input pins (CAPxP and CAPxN) require
capacitors to act as dynamic charge storage for the switched
capacitor input section. Component selection for these capacitors
is critical as the input audio signal appears on or across these
capacitors. A high quality dielectric is recommended for these
capacitors multilayer ceramic, NPO or metal film, PPS for
surface-mounted versions, and polypropylene for through-hole
versions. Indeed, as a general recommendation, high quality
dielectrics should be specified where capacitors are carrying the
input audio signal.
Modulator Direct Input
Figure 23 shows the connection of a single-ended source via an
external single-ended-to-differential converter to the modulator
input of the AD1871. The external amplifier/buffer should have
good slew rate characteristics to meet the dynamic characteristics
of the modulator input that is a switched-capacitor load.
The output of the external amplifier/buffer should be decoupled
from the input capacitors via a 250 W resistor (metal film).
In order to configure the AD1871 for differential input via the
CAPxP and CAPxN pins, the Mux/PGA section must be disabled
by setting the MEL and MER Bits in Control Register III to 1.
120pF
NPO
FERRITE ^10␮F _5.76k⍀
100pF
NPO
5.76k⍀
OP275
5.76k⍀ ^OP275
5.76k⍀
750k⍀
100pF
NPO
237⍀
1nF
NPO
237⍀
100pF
NPO
CAPLN
CAPLP
AD1871
10␮F 100nF
VREF
Figure 23. Direct Connection to Modulator
PGA Input, Single-Ended
Figure 24 shows the connection of a single-ended source to the
PGA section of the AD1871. The PGA section is configured
for single-ended-to-differential conversion. The differential
outputs are connected internally to the CAPxx pins via 250 W
series resistors.
In order to configure the AD1871 for single-ended input, the
Control Registers must be configured as follows:
AD1871
Left Channel
Control Register I = xx0xGGGxxx, where GGG = the Input Gain
(see Table V).
Control Register III = 00xx1x0Sxx, where S = the SE Channel
Selection.
Right Channel
Control Register I = xx0xxxxGGG, where GGG = the Input Gain
(see Table V).
Control Register III = 00xxx1xx0S, where S = the SE Channel
Selection.
100pF
NPO
1nF
NPO
100pF
NPO
FERRITE
600Z
10␮F
100pF
NPO
10␮F 100nF
CAPLN
CAPLP
AD1871
VINLP
VINLN
VREF
Figure 24. Single-Ended Input via PGA Section
PGA Input, Differential
Figure 25 shows the connection of a differential source to the PGA
section of the AD1871. The PGA section is configured as a
differential buffer. The buffered differential outputs are con-
nected internally to the CAPxx pins via a 250 W series resistors.
In order to configure the AD1871 for differential input via the
Mux/PGA, the Control Registers must be configured as follows:
Left Channel
Control Register I = xx0xGGGxxx, where GGG = the Input Gain
(see Table V).
Control Register III = 00xx0x0xxx.
Right Channel
Control Register I = xx0xxxxGGG, where GGG = the Input Gain
(see Table V).
Control Register III = 00xxx0xx0x.
100pF
NPO
1nF
NPO
100pF
NPO
10␮F
2
3
10␮F
1
10␮F 100nF
CAPLN
CAPLP
AD1871
VINLP
VINLN
VREF
Figure 25. Differential Input via PGA Section
REV. 0
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