ISL26132 Datasheet, PDF (19/23 Pages) Intersil Corporation – Low-Noise 24-bit Delta Sigma ADC

ISL26132, ISL26134
Applications Information
Power-up Sequence – Initialization and
Configuration
The sequence to properly power-up and initialize the device are
as follows. For details on individual functions, refer to their
descriptions.
1. A_VDD, D_VDD ramp to specified levels
2. Apply External Clock
3. Pull PDWN High to initiate Reset
4. Device begins conversion
5. SDO/RDY goes low at end of first conversion
OPTIONAL ACTIONS
• Perform Offset Calibration
• Place device in Standby
• Return device from Standby
• Read on-chip Temperature (applicable to ISL26132 only)
Application Examples
WEIGH SCALE SYSTEM
Figure 38 illustrates the ISL26132 connected to a load cell. The
A/D converter is configured for a gain of 128x and a sample rate
of 10Sps. If a load cell with 2mV/V sensitivity is used, the full
scale output from the load cell will be 10mV. On a gain of 128x
and sample rate of 10Sps, the converter noise is 67nV_P-P. The
converter will achieve 10mV/67nV_P-P = 149,250 noise free
counts across its 10mV input signal. This equates to 14,925
counts per mV of input signal. If five output words are averaged
together this can be improved by √5 to yield √5*14925
counts = 33,370 counts per mV of input signal with an effective
update rate of 2 readings per second.
THERMOCOUPLE MEASUREMENT
Figure 39 illustrates the ISL26132 in a thermocouple
application. As shown, the 4.096V reference combined with the
PGA gain set to 128x sets the input span of the converter to
±16mV. This supports the K type thermocouple measurement for
temperatures from -270°C at -6.485mV to +380°C at about
16mV.
If a higher temperature is preferred, the PGA can be set to 64x to
provide a converter span of ±32mV. The will allow the converter
to support temperature measurement with the K type
thermocouple up to about +765°C.
In the circuit shown, the thermocouple is referenced to a voltage
dictated by the resistor divider from the +5V supply to ground.
These set the common mode voltage at about 2.5V. The 5M
resistors provide a means for detection of an open thermocouple.
If the thermocouple fails open or is not connected, the bias
through the 5M resistors will cause the input to the PGA to go to
full scale.
5V
3V
0.1µF
18
1
AVDD
DVDD
VDD
20
-
+
16 _VREF+
9
CAP
0.1µF
10 _CAP
GAIN1
GAIN0
SDO/RDY
19
24
GAIN = 128
SCLK ²³
MICRO
CONTROLLER
ISL26132
22
PDWN
11 _AIN+1
XTALOUT ⁴
12
14
13
AIN-1
AIN+2
AIN-2
XTALIN/CLOCK
SPEED
A0
3
21
8
15 _VREF-
TEMP ⁷
AGND
DGND
GND
17
2, 5, 6
FIGURE 38. WEIGH SCALE APPLICATION
19
FN6954.1
September 9, 2011