Many resistive bridge sensors, such as strain gauges, have
very small full-scale output ranges. For these sensors, the
measurement resolution obtainable without additional ampli-
fication can be low. For example, if the bridge sensor output
is ±20mV, the ADS1100 outputs codes from approximately
–873 to +873, resulting in a best-case resolution of around 11
bits. If higher resolution is required, it is best to supply an
external instrumentation amplifier to bring the signal to full
scale.
If the ADS1100 is driven by an op amp with high voltage
supplies, such as ±12V, protection should be provided, even
if the op amp is configured so that it will not output out-of-
range voltages. Many op amps seek to one of the supply rails
immediately when power is applied, usually before the input
has stabilized; this momentary spike can damage the ADS1100.
Sometimes this damage is incremental and results in slow,
long-term failure—which can be distastrous for permanently
installed, low-maintenance systems.
If you use an op amp or other front-end circuitry with the
ADS1100, be sure to take the performance characteristics of this
circuitry into account. A chain is only as strong as its weakest link.
ADVICE
The ADS1100 is fabricated in a small-geometry low-voltage
process. The analog inputs feature protection diodes to the
supply rails. However, the current-handling ability of these
diodes is limited, and the ADS1100 can be permanently
damaged by analog input voltages that remain more than
approximately 300mV beyond the rails for extended periods.
One way to protect against overvoltage is to place current-
limiting resistors on the input lines. The ADS1100 analog
inputs can withstand momentary currents of as large as
10mA.
LAYOUT TIPS
PCB layout for the ADS1100 is relatively undemanding.
16-bit performance is not difficult to achieve.
Any data converter is only as good as its reference. For the
ADS1100, the reference is the power supply, and the power
supply must be clean enough to achieve the desired perfor-
mance. If a power-supply filter capacitor is used, it should be
placed close to the VDD pin, with no vias placed between the
capacitor and the pin. The trace leading to the pin should be as
wide as possible, even if it must be necked down at the device.
The previous paragraph does not apply to the I2C ports,
which can both be driven to 6V regardless of the supply.
ADS1100
SBAS239B
15
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