Datasheet

Table Of Contents

JMP7

CP SCK

NO CLK

SPI_SCK

C11

R11

C13

C12

R10

XTR108 I

330pF 30.1k

36.5k

1nF

0.01PF

JMP6

BYPASS

BUF

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General Operating Tips

6 General Operating Tips

6.1 Voltage Output Mode

In many applications, it is desirable to bypass the FET and connect the XTR108 directly to the power

supply. This configuration can be completed by positioning jumpers JMP2 and JMP3 in the Bypass

position.

NOTE: You must be careful in this mode to not apply an overvoltage to the XTR108. The XTR108

maximum supply voltage is 5.5 V. The voltage drop across D1 reduces the XTR108 supply

voltage by approximately 0.7 V. To get an accurate idea of the XTR108 supply voltage while

in this mode, you should measure at the XTR108 V

pin.

6.2 Charge Pump

When the XTR108 is in voltage-output mode, a small negative voltage must be applied to the XTR108 I

pin to prevent the input offset voltage of the XTR108 current output stage from turning on the output

stage. A simple discrete charge pump circuit, shown in Figure 33, can be used to generate this small

negative voltage. The charge pump input is the SPI clock to the XTR108.

Figure 33. Discrete Charge Pump on Sensor Interface Board

For this circuit to work, the XTR108 clock must toggle continuously. This toggling can be accomplished by

keeping the XTR108 in continuous EEPROM read mode. JMP6 can be used to connect or disconnect an

external buffer to the XTR108 clock signal. More information regarding this charge pump is located in the

XTR108 data sheet and the XTR108 Quick Start System Reference Guide.

SBOU123–March 2012 XTR108EVM-USB Evaluation Board and Software Tutorial

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