User manual

Product Overview
 2010 Microchip Technology Inc. DS51918A-page 11
FIGURE 1-3: Gain Selection Options on MCP3421 Weight Scale Demo Board.
1.3 LOAD CELL
The MCP3421 device with a maximum PGA gain setting (8x) can detect an input signal
level as low as 2 µV. Therefore, it can detect the input with an accuracy of 1g. When
measuring such a low input signal level, to control the noise level at the input pins is
very important. Therefore, using low-noise op amp at the signal conditioning circuit is
necessary. In this demo board, MCP6V07 auto-zeroed op amp is used. This op amp
has the offset voltage less than ±2 µV, and drifts less than ±50 nV/C.
Additional improvement in signal-to-noise ratio can be achieved by doing more averag-
ing in digital domain. The user can tell the difference by selecting the data average
options 1 or 4. The results are shown in the PC GUI.
TABLE 1-1: JUMPERS POSITIONS VS. FUNCTION
Symbol Jumpers Positions Function
xPGA
(1)
J4 and J5 connected to J19
J10 and J13 are ON
J8, J9, J11 and J12 are disconnected (
Figure 1-3a)
This combination evaluates the ADC perfor-
mance. The on-board op amp is bypassed.
xGLo
(1)
J4 and J5 connected to GND
J9, J11 and J12 are ON
J8, J10 and J13 are disconnected (
Figure 1-3b)
This combination evaluates the influence of the
extra gain added by the MCP6V07 device.
xGHi
(1)
J4 and J5 connected to GND
J8, J11 and J12 are ON
J9, J10 and J13 are disconnected (
Figure 1-3c)
This combination evaluates the influence of the
extra gain added by the MCP6V07 device, but
with higher extra gain.
Note 1: x indicates the selected averaging factor (1 or 4).
Note: This demo board uses a low-cost load cell for demonstration purposes. The
load cell can measure up to 2 Kg with 1 mV/V sensitivity. The load cell
output is connected to the demo board PCB through J18 and J19
connectors. The user can also connect their own load cell instead.