Weigh Scale Circuit Diagrams
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The circuits shown here utilize ultralow noise 24-bit Sigma-Delta ADCs with internal PGA and drivers, to design precision weigh scale systems spanning low- and high-speed applications. The sigma-delta ADCs simplify the weigh scale design because most of the system building blocks are included on the chip. An appropriate low drop-out linear regulator is specified for use, and the benefits of each circuit are described below. Visit the individual circuit note pages to learn more...

Circuit Benefit Devices Used Detailed Circuit Note
One of two lowest noise from this collection, with RMS noise = 8.5 nV @ 4.7 Hz. Supports ac and dc excitation of the loadcell. AD7195: 4.8 kHz, Ultralow Noise, 24-Bit Sigma-Delta ADC with PGA and AC Excitation, $6.901
ADP3303: 200 mA low drop-out linear regulator, $1.11		 CN01552(shown below)
One of two lowest noise from this collection, with RMS noise = 8.5 nV @ 4.7 Hz. Supports dc excitation of the loadcell only. AD7190: Pin programmable low noise 24-bit Σ-Δ ADC with PGA, $5.901
ADP3303: 200 mA low drop-out linear regulator, $1.11		 CN0102
Slightly higher noise level (11 nV @ 4.7 Hz), but lower in power, cost. AD7192: 4.8 kHz Ultra-Low Noise 24-Bit Σ-Δ ADC with PGA, $4.90
ADP3303: 200 mA low drop-out linear regulator, $1.11 		CN0119
Easier circuit to implement due to pin programmability. RMS Noise = 15 nV @ 10 Hz AD7191: 24-bit pin-programmable Σ-Δ, $3.80
ADP3303: 200 mA low drop-out linear regulator, $1.11		 CN0118
Low power and low noise: RMS Noise = 49 nV @ 10 Hz and power at 330 uA typical AD7780: Pin Programmable 24-Bit Σ-Δ ADC with PGA and pin reset, $2.70
ADP3303: 200 mA low drop-out linear regulator, $1.11		 CN01072
Also low power (330uA) and low noise (49nV rms noise @10Hz). Plus, lowest price in collection. AD7781: Pin Programmable 20-Bit Σ-Δ ADC with PGA and pin reset, $1.95
ADP3303: 200 mA low drop-out linear regulator, $1.11 		CN01082
1Prices shown are in USD in quantities greater than 1000, recommended lowest grade resale, FOB U.S.A.
2Design and Integration files are available for this circuit


Precision Weigh Scale Design Using a 24-Bit Sigma-Delta
ADC with Internal PGA and AC Excitation (CN0155)

Weigh Scale System Using the AD7195 with AC Excitation
(Simplified Schematic: All Connections Not Shown)
Circuit Description

With ac excitation, the polarity of the excitation voltage to the load cell is switched using external MOSFETs. Consecutive values are then averaged, which removes dc induced offsets. The AD7195 contains the internal logic to control the switching of the external MOSFETs. The driver signals from the AD7195 are precisely timed and nonoverlapping, which ensures that no short circuit occurs when switching the polarity of the bridge drive voltage. Pull-up and pull-down 1 MO resistors are connected to ACX2 and ACX2 to prevent short-circuiting during power-up. The AD7195 provides an integrated solution for ac excited load cells. The AD7195 can accept an inverted reference voltage, which is required when the excitation voltage to the load cell is inverted. The AD7195 synchronizes the ac excitation with the conversions and performs the averaging.
Continue reading the full circuit note...
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