Cirrus Logic CDB4244 - Manual (Page 15)

DS900DB2 15
CDB4244
4 ANALOG INPUT FILTERS
The CDB4244 makes it possible to realize two different analog input filter options for interfacing with the CS4244: a
convenient active single-ended-to-differential filter to showcase maximum analog input performance, and an option-
al passive differential filter to minimize component count. Refer to AN340: CS4234/4244 Filter Considerations for a
detailed discussion of the theory and design decisions behind each filter. The following sections highlight the use
cases of each filter.
4.1 Active Single-Ended-to-Differential Input Filter
By default, the CDB4244 is configured with an active single-ended-to-differential input filter network on each
input channel to allow a simple single-ended signal to be connected to each of the board’s analog inputs.
The filter network converts the single-ended signal into a differential signal to interface with the differential
analog inputs of the CS4244, while minimizing source impedance in order to achieve maximum analog input
performance.
Figure 6 highlights the signal flow of this filter.
Figure 6. Active Single-Ended-to-Differential Input Filter Block Diagram
Use of this filter network requires a dual-rail ±12-V power supply to power the operational amplifiers em-
ployed in the design. Note that the amplitude of the single-ended input signal is doubled by the differential
conversion. As such, the full-scale input voltage that can actually be applied at the input of the filter is 6 dB
below the full-scale input voltage specified in the CS4244 data sheet.
4.2 Passive Differential Input Filter
Alternatively, each input channel of the CDB4244 can be configured with a passive differential input filter
network to reduce component count at the cost of performance. In this configuration, a differential input sig-
nal is connected to each analog input of the CDB4244, filtered through a passive network, and presented
directly to the CS4244 analog inputs.
Figure 7 highlights the signal flow of this filter.
Figure 7. Passive Differential Input Filter Block Diagram
Because no active circuit elements are used in this design, the dual-rail ±12-V power supply can be removed
completely if no active output filters are in use (refer to
Section 5.1 on page 17 ). If only the +5-V power sup-
ply remains in use, the CDB4244 can be powered solely from the PC USB port when the board power is
configured as specified in
Table 4 on page 21 .
Single-Ended Input
AIN+
AIN-
Non-Inverting
Buffer
Inverting
Buffer
CS4244
Differential Input
AIN+
AIN-
Passive Filter
Network
CS4244