DN276 - LTC1564: A Digitally Tuned Antialiasing / Reconstruction Filter Simplifies High Performance DSP Design

LTC1564: A Digitally Tuned Antialiasing/Reconstruction Filter
Simplifies High Performance DSP Design – Design Note 276
Max W. Houser and Philip Karantzalis
Introduction
Typically an analog antialiasing filter is used to band-limit
wideband signals at the input of an analog-to-digital
converter. In addition, as the converter’s sampling
rate changes, an antialiasing filter’s passband should
increase or decrease accordingly. A frequency-tunable
analog filter for a high resolution converter requires a
large number of expensive precision components. With
the LTC®1564, designers of data acquisition instruments
and digital signal processing (DSP) systems have a low
noise, continuous-time, “brick wall” lowpass filter with
digital control of the corner frequency fC, (fC range 10kHz
to 150kHz in 10kHz steps). The LTC1564 also includes
a digitally programmable gain amplifier (PGA, 1V/V to
16V/V in 1V/V steps). A simple, on-chip, latching digital
interface controls corner frequency and gain settings.
The LTC1564 is in a small 16-pin SSOP and operates
from a supply voltage of 2.7V to 10.5V total (single or
split supplies).
Filtering Performance and Operation
The LTC1564 is a high resolution filter with a rail-to-rail
output. The 8th order lowpass response with two stopband notches gives approximately 100dB attenuation
at 2.5 times fC, making it suitable for high resolution
antialiasing filtering. Despite the high filter order, the
wideband noise is only 33μVRMS (typical) at a 20kHz
corner frequency and unity gain, which is 100dB below
the rail-to-rail maximum signal level for ±5V supplies.
The output-referred noise rises only slightly at higher
gain settings. At the maximum 24dB (16V/V) gain
setting, the same 20kHz response just quoted has an
output noise level of 40μVRMS (or an input-referred
noise of 2.5μVRMS). Gain control in the LTC1564 is an
integral part of the filter, using a proprietary method
that deliberately minimizes the total noise. This feature
is very difficult to achieve with separate variable gain
amplifiers and filter circuits. The LTC1564 satisfies a
demand for lowpass filters with roughly “100-100-100”
performance: 100dB stopband attenuation, 100dB
signal-to-noise ratio (SNR) and 100kHz bandwidth.
01/02/276_conv
You do not have to be a filter expert or analog designer
to use the LTC1564. There are only three analog pins:
Input, Output and a half-supply reference voltage point,
AGND (Figure 1). The other pins are digital controls and
power supply. The LTC1564 is an instrument in a box with
analog input and output jacks and two rotary switches
labeled “Frequency” and “Gain.” The frequency setting
“F” and gain setting “G” are 4-bit codes entered through
the F and G digital input pins (Table 1). In addition, setting the F code to 0000 engages a “mute” state where
the filter remains fully powered but the gain is a hard
zero (typically –100dB). Logic levels for the LTC1564
digital inputs are nominally rail-to-rail CMOS (where a
logic 1 is V+ and a logic 0 is 0V for single 3V or 5V or
dual ±5V supply operation).
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
ANALOG
IN
V+
GAIN CODE
16
15
14
13
12
IN AGND V + RST G3
10
11
9
G2
G1
G0
F2
F1
F0
LTC1564
OUT V –
1
ANALOG
OUT
2
V–
CS/
EN HOLD F3
3
4
5
6
7
8
FREQUENCY CODE
POSITIVE AND NEGATIVE
SUPPLIES CAN BE FROM
1.35V TO 5.25V EACH
DN276 F01
Figure 1. Dual Power Supply Circuit
Table 1. Programming the Corner Frequency and Gain of
the LTC1564
F3 F2 F1 F0 G3 G2 G1 G0 MODE
0 0 0 1 0 0 0 0 fC = 10kHz,
Passband Gain = 1V/V (0dB)
1 1 1 1 0 0 0 0 fC = 150kHz,
Passband Gain = 1V/V (0dB)
0 0 0 1 1 1 1 1 fC = 10kHz,
Passband Gain = 16V/V (24dB)
0 0 0 0
Don’t Care Mute State, Zero Gain
wave was preamplified to 4.5VP-P by the LTC1564 to
nearly span the input range of the LTC1608 ADC. The
LTC1564 is set for a cutoff frequency of 50kHz and a
gain of 16V/V and the sampling frequency of the ADC
is 204.8kHz. Total harmonic distortion (THD) is 86dB
down and the dynamic range is 109dB (since the filter
noise does not increase with gain, the programmable
filter gain extends the dynamic range beyond the unity
gain range).
Application Example: 2-Chip “Universal”
DSP Front End
In Figure 2, an LTC1564 filter drives an LTC1608
16-bit 500ksps analog-to-digital converter (ADC) for
a highly flexible, complete 16-bit analog-to-digital
signal interface with variable gain, variable sampling
rate and variable analog bandwidth up to 150kHz. The
LTC1564’s frequency-setting “F” code and the rate of
sampling controlled by the LTC1608’s CONVST input
(Pin 31) set signal bandwidth and antialiasing filtering.
Conclusion
In addition to providing high resolution antialiasing, the
LTC1564 is useful as a reconstruction filter that eliminates the nonessential high frequency signal spectrum
at the output of a digital-to-analog converter (DAC). A
simple, compact, economical and high performance
digital signal processing and generating system hardware requires only two LTC1564, an ADC and a DAC.
As an example, with the LTC1564 passband corner (fC)
set to 100kHz, with a sampling rate (fS) of 500ksps
by the LTC1608 ADC, provides 100dB of antialiasing
protection at the critical analog folding frequency of
fS/2, or 250kHz. Another independent option is to
sample at a rate (fS) that is lower than 5 • fC. This will
move the folding frequency (fS/2) down from 2.5 • fC to
somewhere within the analog filter’s roll-off band, where
the filter’s rejection will not be as high as 100dB. This
reduces the antialias rejection for signals at and above
fS/2, but still provides sufficient antialias protection in
many applications, particularly if, as is often true, the
aliasable signals at and above fS/2 have lower levels
than the desired signals at and below fC. The circuit
of Figure 2 can accommodate either or both of these
options by suitable choice of ADC sampling rate and
filter F code.
0
–20
AMPLITUDE (dB)
–40
–5V
5V
–120
–140
0
+
0.1μF
1μF
2.2μF
EN V–
IN
25.6
51.6
76.8
FREQUENCY (kHz)
10Ω
36
AVDD
VREF
1μF
5V
5V
35
AVDD
LTC1608
LTC1564
4 REFCOMP
AV
7.5k
2.5V
REF
1.75X
+
1μF
9
10
DVDD
DGND
CONTROL
LOGIC
AND
TIMING
INPUT
–
102.4
Figure 3. FFT Plot of the Digital Output of
Figure 2's Circuit
3
V+
–80
–100
Figure 3 shows a measured FFT spectrum of the digital
output of Figure 2’s circuit. A 40kHz, 100mVRMS sine
0.1μF
–60
SHDN 33
CS 32
μP
CONTROL
LINES
CONVST 31
RD 30
BUSY 27
22μF
OVDD 29
249Ω
1% METAL FILM
FC
1 AIN+
OUT
249Ω
1% METAL FILM
C1*
AGND
CS/
HOLD
RST
F
2 AIN–
+
–
OGND 28
16-BIT
SAMPLING
ADC
AGND
G
5
FILTER CONTROL
*C1 IS A 1000pF NPO, SURFACE MOUNT DEVICE
PLACE AS CLOSE AS POSSIBLE TO THE LTC1608 INPUT PINS
5V OR
3V
1μF
AGND
OUTPUT
BUFFERS
B15 TO B0
AGND
6
DIFFERENTIAL
ANALOG INPUT
±2.5V
7
D15 TO D0
16-BIT
PARALLEL
BUS
11 TO 26
AGND VSS
8
34
DN276 F02
1μF
–5V
Figure 2. A Universal DSP Front End
Data Sheet Download
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