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4
nodes. In these application, operation should improve if a
system calibration is used.
AN EXTERNAL VS INTERNAL 2.5V REFERENCE
An external reference is recommended for the circuit design
where 23 bits
rms effective resolution is the design goal. The
recommended circuit configuration is shown in Figure 4. In
this circuit a REF1004-2.5, 2.5V reference from Burr-Brown
is used. This same reference chip along with the support
circuitry is implemented on the demonstration fixture for the
ADS1210 and ADS1211, (DEM-ADS1210/11). The internal
reference for the device will usually provide at most
20 bits
rms, regardless of the other measures to achieve
optimum performance. Since the ADS1210 and ADS1211
reference is a CMOS reference, it has more noise than an
equivalent bipolar reference. The effective resolution graphs
included in the product data sheet for the ADS1210/11
products were generated with an external reference
(REF1004-2.5).
SPECIAL CARE WITH THE INPUT PINS
In terms of the differential analog inputs, there is one pair
inputs with the ADS1210 and four pairs with the ADS1211.
Three techniques can be used to reduce the input noise to the
converter at these pins. A primary consideration is to make
FIGURE 4. This is the recommended external reference circuit for the ADS1210 and ADS1211 when an effective resolution
of 23 bits rms is required.
A
IN
4P
A
IN
4N
A
IN
3P
A
IN
3N
A
IN
2P
A
IN
2N
A
IN
1P
A
IN
1N
VBIAS
AGND
AV
DD
REF
IN
REF
OUT
22
23
24
1
2
3
4
5
7
6
19
21
20
XOUT
XIN
MODE
ADS1211
R13
0
C
2
10μF
U3
REF1004
C9
1μF
C15
0.1μF
+5V
R8
50k
1
J4
DRDY
SDOUT
SDIO
SCLK
DSYNC
CS
17
16
15
14
9
8
DV
DD
DGND
12
13
10
18
11
Analog Power
Supply
EFFECTIVE
RESOLUTION
(
μ
Vrms)
EFFECTIVE
RESOLUTION
(Bits rms)
TEST CONDITIONS
Short analog and DUT digital ground and power
at the chip by shorting pin 6 (AGND) to pin 12
(DGND) and pin 19 (AV
DD
) to pin 13 (DV
DD
).
18
18.8
Short analog and DUT digital ground at the
chip by shorting pin 6 (AGND) to pin 12 (DGND).
18
18.8
Short analog and DUT digital power at the chip
by shorting pin 19 (AV
DD
) to pin 13 (DV
DD
).
18
18.8
Short analog and DUT digital ground and power
at the edge of the board by inserting a shorting
bar in R1 (for ground short) and CRN2
(for power plane short).
5
20.6
Short analog and DUT digital ground at the
edge of the board by inserting a shorting bar
in R1.
5
20.6
Short analog and DUT digital power at the edge
of the board by inserting a shorting bar CRN2.
5
20.6
Connect the power to the board from the bench
power supply with four one inch wires.
1
23
TABLE IV. ADS1211, X
IN
= 10MHz, data rate = 60Hz,
Turbo Mode = 16, PGA = 1, expected number
of effective bits rms = 23.
converter disconnects the inputs from the input pins and
performs the self calibration. Once the device goes into
normal operation, the noise from a non-perfect sine wave
oscillator can be coupled into the high impedance input