WM0834, WM0838
Functional Description (continued)
WM0834 MUX Addressing
Channel Number
MUX Address
ODD/EVEN
SGL/DIF
Select bit
0
1
2
3
Differential MUX Mode (Between adjacent pairs of points)
0
0
0
0
0
0
1
1
0
1
0
1
+
-
-
+
-
-
+
+
+
Single Ended MUX Mode (between selected input(s) and AGND)
1
1
1
1
0
0
1
1
0
1
0
1
+
+
+
Note: Analogue common input 'COM' used with single ended mode is internally tied to AGND
All internal registers are cleared when CS is high. To
initiate another conversion, CS must make a high to low
transition and MUX address assignments input to DI.
Analogue Inputs
While sampling the analogue inputs, short spikes of
current enter a "+" input and flow out of the corresponding
"-" input at the clock edges during conversion. This
current does not cause errors as it decays rapidly and the
internal comparator is strobed at the end of a clock period.
Care should be exercised if bypass capacitors are used at
the inputs as an apparant offset error can be caused by the
capacitor averaging the input current and developing a volt-
age across the source resistance. Bypass capacitors should
not be used with a source resistance greater than 1kΩ.
The DI inpit and DO output can be tied together and
controlled via a bidirectional processor I/O bit line.
Reference Input
The analogue input voltage range Vmax to Vmin for
differential and pseudo-differential input is defined by the
voltage applied to the reference input with respect to
AGND.
In considering error sources, input leakage current will also
cause a voltage drop across the source resistance and
hence, high impedance sources should be buffered.
WM0834/8 can be used in ratiometric appliacations or
those requiring absolute accuracy. A ratiometric input is
typically the Vcc and is the same supply used to power
analogue input circuitry and sensors. In such systems
under a given input condition, the same code will be
output with variations in supply voltage, because the same
ratio change occurs in both the analogue and reference
input to the A/D. When used in applications requiring ab-
solute accuracy, a suitable time and temperature stable
voltage reference source should be used.
In differential mode, there is a 1/2 clock period interval
between sampling the "+" and the "-" inputs. If there is a
change in common mode voltage during this interval, an
errorcould notionally result.
For a sinusoidal common mode signal, the error is given
by:
The voltage source used to drive the reference input should
be capable of driving the 2.4kΩ typical of the SAR resistor
ladder. The maximum input voltage to the reference input
is the Vcc supply voltage. The minimum can be as least
as low as 1V to allow for direct conversion of sensor
outputs with output voltage ranges less than 5V.
VERROR = VPEAK (2πfCM) (1/(2fCLK))
Where
VPEAK = peak common mode voltage
fCM = common mode signal frequency
fCLK = clock frequency
Wolfson Microelectronics
11
WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
