CN-0216

Circuit Note

Rev. 0 | Page 2 of 5

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.

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.