CN0216 Datasheet, PDF (2/5 Pages) Analog Devices – Devices Connected

CN-0216
CIRCUIT DESCRIPTION
Figure 2 shows the actual test setup. For testing purposes, a
6-wire Tedea-Huntleigh 505H-0002-F070 load cell is used.
Current flowing through a PCB trace produces an IR voltage
drop, and with longer traces, this voltage drop can be several
millivolts or more, introducing a considerable error. A 1 inch
long, 0.005 inch wide trace of 1 oz copper has a resistance of
approximately 100 mΩ at room temperature. With a load
current of 10 mA, this can introduce a 1 mV error.
A 6-wire load cell has two sense pins, in addition to the
excitation, ground, and two output connections. The sense pins
are connected to the high side (excitation pin) and low side
(ground pin) of the Wheatstone bridge. The voltage developed
across the bridge can be accurately measured regardless of the
voltage drop due to wire resistance. In addition, the AD7791
accepts differential analog inputs and a differential reference as
well. These two sense pins are connected to the AD7791
reference inputs to create a ratiometric configuration that is
immune to low frequency changes in the power supply
excitation voltage. The ratiometric connection eliminates the
need for a precision voltage reference.
Unlike a 6-wire load cell, a 4-wire load cell does not have sense
pins, and the ADC differential reference pins are connected
directly to the excitation voltage and ground. With this
connection, there exists a voltage difference between the
excitation pin and the reference pin on the ADC due to wire
resistance. There will also be a voltage difference on the low side
(ground) due to wire resistance. The system will not be
completely ratiometric.
The Tedea-Huntleigh 2 kg load cell has a sensitivity of 2 mV/V
and a full-scale output of 10 mV when the excitation voltage is
5 V. A load cell also has an offset, or TARE, associated with it.
In addition, the load cell also has a gain error. Some customers
use a DAC to remove or null the TARE. When the AD7791 uses
a 5 V reference, its differential analog input range is equal to
±5 V, or 10 V p-p. The circuit in Figure 1 amplifies the load cell
output by a factor of 375 (1 + 2R1/RG), so the full-scale input
range referred to the load cell output is 10 V/375 = 27 mV p-p.
This extra range relative to the 10 mV p-p load cell full-scale
signal is beneficial as it ensures that the offset and gain error of
the load cell do not overload the ADC’s front end.
The low-level amplitude signal from the load cell is amplified by
two ADA4528-1 zero-drift amplifiers. A zero-drift amplifier, as
the name suggests, has a close to zero offset voltage drift. The
amplifier continuously self-corrects for any dc errors, making it
as accurate as possible. Besides having low offset voltage and
drift, a zero-drift amplfier also exhibits no 1/f noise. This
important feature allows precision weigh scale measurement at
dc or low frequency.
Circuit Note
The two ADA4528-1 op amps are configured as the first stage of
a three op amp instrumentation amplifier. A third op amp
connected as a difference amplifier would normally be used for
the second stage, but in the circuit of Figure 1, the differential
input of the AD7791 performs this function.
The gain is equal to 1 + 2R1/RG. Capacitors C1 and C2 are
placed in the feedback loops of the op amps and form 4.3 Hz
cutoff frequency low-pass filters with R1 and R2. This limits the
amount of noise entering the sigma-delta ADC. C5 in
conjunction with R3 and R4 form a differential filter with a
cutoff frequency of 8 Hz, which further limits the noise. C3 and
C4 in conjunction with R3 and R4 form common-mode filters
with a cutoff frequency of 159 Hz.
The ADP3301 low noise regulator powers the AD7791,
ADA4528-1, and the load cell. In addition to decoupling
capacitors, a noise reduction capacitor is placed on the regulator
output as recommended in the ADP3301 data sheet. It is
essential that the regulator is low noise, because any noise on
the power supply or ground plane introduces noise into the
system and degrades the circuit performance.
Figure 2. Weigh Scale System Setup Using the AD7791
The 24-bit sigma-delta ADC AD7791 converts the amplified
signal from the load cell. The AD7791 is configured to operate
in the buffered mode to accommodate the impedance of the
R-C filter network on the analog input pins.
Figure 3 shows the AD7791’s rms noise for different output data
rates. This plot shows that the rms noise increases as the output
data rate increases. However, the device maintains good noise
performance over the complete range of output data rates.
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