ADS1255IDB Burr-Brown Corporation, ADS1255IDB Datasheet - Page 28

• Manufacturer Part Number: ADS1255IDB
• Description: Very Low Noise/ 24-Bit Analog-to-Digital Converter
• Manufacturer: Burr-Brown Corporation

ADS1255
ADS1256
SBAS288D − JUNE 2003 − REVISED AUGUST 2004
APPLICATIONS INFORMATION
GENERAL RECOMMENDATIONS
The ADS1255 and ADS1256 are very high-resolution A/D
converters. Getting the optimal performance from them
requires careful attention to their support circuitry and
printed circuit board (PCB) design. Figure 25 shows the
basic connections for the ADS1255. It is recommended to
use a single ground plane for both the analog and digital
supplies. This ground plane should be shared with the
bypass capacitors and analog conditioning circuits.
However, avoid using this ground plane for noisy digital
components such as microprocessors. If a split ground
plane is used with the ADS1255/6, make sure the analog
and digital planes are tied together. There should not be a
voltage difference between the ADS1255/6 analog and
digital ground pins (AGND and DGND).
As with any precision circuit, use good supply bypassing
techniques. A smaller value ceramic capacitor in parallel
with a larger value tantalum or a larger value low-voltage
ceramic capacitor works well. Place the capacitors, in
particular the ceramic ones, close to the supply pins. Run
the digital logic off as low of voltage as possible. This helps
reduce coupling back to the analog inputs. Avoid ringing
on the digital inputs. Small resistors ( 100 ) in series with
28
VIN
VIN
2.5V
P
N
49.9
49.9
301
301
+5V
+3.3V
10 F
47 F
10 F
Figure 25. ADS1255 Basic Connections
0.1 F
0.1 F
0.1 F
0.1 F
100pF
100pF
10
1
2
3
4
5
6
7
8
9
AVDD
AGND
VREFN
VREFP
AINCOM
AIN0
AIN1
SYNC/PDWN
RESET
DVDD
the digital pins can help by controlling the trace
impedance. When not using the RESET or SYNC/PDWN
inputs, tie directly to the ADS1255/6 DVDD pin.
Pay special attention to the reference and analog inputs.
These are the most critical circuits. On the voltage
reference inputs, bypass with low equivalent series
resistance (ESR) capacitors. Make these capacitors as
large as possible to maximize the filtering on the reference.
With the outstanding performance of the ADS1255/6, it is
easy for the voltage reference to limit overall performance
if not carefully selected. When using a stand-alone
reference, make sure it is very low noise and very low drift.
Ratiometric measurements, where the input signal and
reference track each other, are somewhat less sensitive,
but verify the reference signal is clean.
Often times, only a simple RC filter (as shown in Figure 25)
is needed on the inputs. This circuit limits the
high-frequency noise near the modulator frequency; see
the Frequency Response section. Avoid low-grade
dielectrics for the capacitors to minimize temperature
variations and leakage. Keep the input traces as short as
possible and place the components close to the input pins.
When using the ADS1256, make sure to filter all the input
channels being used.
ADS1255
XTAL1/CLKIN
D0/CLKOUT
XTAL2
DGND
DOUT
DRDY
SCLK
DIN
CS
D1
20
19
18
17
16
15
14
13
12
11
7.68MHz
100
100
100
18pF
18pF
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