Cirrus Logic AN31 - Manual (Page 21)

excitation. In external excitation mode, the BX1
pin of the converter is an input and is used to
determine the polarity of the excitation. The
phase of the signal at BX1 controls the phase of
the internal detection circuitry. Each time the
polarity of the excitation is changed, the
converter needs six conversion word periods for
the internal digital filter to accurately settle on
the input signal. To yield a proper result, the
sixth conversion word for each of the excitation
phases will need to be averaged together. For
optimum throughput, the excitation polarity
should be changed when the DRDY signal falls.
The on-chip calibration features may not be
usable directly when operating in this manner,
but the user microcontroller can manipulate the
gain and offset registers in the converter to
optimize the the offset and gain adjustments for
optimum operation. If the bridge polarity is
reversed every six conversion words, an output
result can be computed every twelve filter
cycles. This will yield an effective conversion
update rate of about four updates per second
(XIN = 4.096 MHz).
CS5516 or CS5520 and a 1 mV/V
AC-Excited Load Cell
Metal film or metal foil strain gages are
generally configured to yield a sensitivity of
2 mV/V or 3 mV/V from a load cell. A load cell
may be used at 1/2 or 1/3 its rated capacity to
allow it to have greater overload capacity. A
designer may trade sensitivity for overload
capability. For example, using a 2 mV/V load
cell at 1/2 capacity yields a 1 mV/V sensitivity,
but with greater ruggedness. The lower
sensitivity results in less output signal for a
given excitation. The usable portion of the
output signal may be further reduced because the
load cell may be part of a system where the pan
weight consumes a good portion of the signal
span of the load cell output. For example, a scale
designed to weigh 10 Kg (22 lbs.) may have a
pan weight which weighs 5 Kg. (11 lbs.) and
therefore the pan weight consumes half of the
signal span out of the load cell. The application
may require protection against high impact, such
as when the items being weighed are dropped on
the scale. A 2 mV/V load cell may be derated
which results in lower output sensitivity
(1 mV/V or so) to allow greater impact capacity
for the load cell.
Figure 16 illustrates such an application. The
signal to measured from the bridge is only 5 mV
over the measurement range (the pan weight
consumes 5 mV of the load cell span). The
offset calibration capability of the
CS5516/CS5520 converter can readily remove
the offset due to the pan weight. If the converter
was configured to measure the 5 mV signal
without the additional buffer amplifier, the 5 mV
signal would only use part of the converter’s
span. For example, if the VREF voltage is
reduced to 2.0 V and the PGA gain inside the
converter is set to 8, the input span expected by
the converter would be 2.0/(25 X 8) = 10 mV.
To calibrate the converter with only a 5 mV
signal would force the gain register to a value
outside the recommended range (1.2 to 0.8).
This situation can be overcome by using an
external buffer amplifier made up of two OP-27
op amps. The VREF voltage for the converter is
set to 3.33 V by using three equal resistors for
R1, R2, and R3. The PGA gain is set to 1 which
makes the input sensitivity at the AIN pins of
the converter to be 3.33/(25 X 1) = 133 mV. The
buffer amplifies the usable portion of the load
cell output signal (5 mV) by a gain of 26 to
yield an input to the converter of 130 mV.
Stability of the gain resistors is important but
tight initial tolerance is not needed as the gain
calibration feature of the CS5516/CS5520 can
accommodate up to
±
20% gain scaling. AC
excitation removes the offset of the OP-27s.
The circuit is operated with the load cell excited
with a 1 kHz bridge drive frequency. When
operating in bipolar mode, the CS5516 converter
will yield about 27,000 noise-free counts over
Bridge Transducer Digitizer Circuits
AN31REV3 21