Increasing the iMEMS Accelerometer’s Output Scale Factor Figure 15 shows the basic connections for using an external buffer amplifier to increase die output scale factor. The output multiplied by the gain of the buffer, which is simply the value of resistor R3 divided by RI. Choose a convenient scale factor, keeping in mind that the buffer pin not only amplifies the signal, but my noise or drift as well. Too much pin can also cause the buffer to saturate and clip the output waveform. Note that the “+” input of the external op amp uses the offset null pin of. | ADXL150 ADXL250 Increasing the i MEMS Accelerometer s Output Scale Factor Figure 15 shows the basic connections for using an external buffer amplifier to increase die output scale factor. The output multiplied by the gain of the buffer which is simply the value of resistor R3 divided by RI. Choose a convenient scale factor keeping in mind that the buffer pin not only amplifies the signal but my noise or drift as well. Too much pin can also cause the buffer to saturate and clip the output waveform. Note that the input of the external op amp uses the offset null pin of the ADXL150 ADXL250 as a reference biasing the op amp at midsupply saving two resistors and reducing power consumption. The offset null pin connects to the VS 2 reference point inside the accelerometer via 30 kQ so it is important not to load this pin with more dim a few microamps. It is important to use a single-supply or rail-to-rail op amp for the external buffer as it needs to be able to swing close to the supply and ground. The circuit of Figure 15 is entirely adequate for many applications but its accuracy is dependent on the pretrimmed accuracy of the accelerometer and this will vary by product type and grade. For the highest possible accuracy an external trim is mended. As shown by Figure 20 this consists of a potentiometer Rla in series with a fixed resistor Rlb. Another to select resistor values after measuring the device s scale see Figure 17 . AC Coupling If a dc gravity response is not required for example tion measurement applications ac coupling can be between the accelerometer s output and the external op input as shown in Figure 16. The use of ac coupling eliminates my zero g drift and allows the maximum amp gain without clipping. Resistor R2 and capacitor C3 together form a high whose corner frequency is 1 2 x R2 C3 . This filter the signal from the accelerometer by 3 dB at the and it will continue to reduce it at a rate of 6 20 dB per decade for signals below the corner frequ .
