8.3.10.2.1 DC Lead-Off

In this method, the lead-off excitation is with a dc signal. The dc excitation signal can be chosen from either an external pull-up or pull-down resistor or a current source or sink, as shown in Figure 38. One side of the channel is pulled to supply and the other side is pulled to ground. The internal current source and current sink can be swapped by setting the FLIP1 and FLIP2 bits in the LOFF_SENS register. In case of current source or sink, the magnitude of the current can be set by using the ILEAD_OFF[1:0] bits in the LOFF register. The current source or sink gives larger input impedance compared to the 10-MΩ pull-up or pull-down resistor.

Figure 38. DC Lead-Off Excitation Options

Sensing of the response can be done either by looking at the digital output code from the device or by monitoring the input voltages with an on-chip comparator. If either of the electrodes is off, the pull-up resistors and the pull-down resistors saturate the channel. By looking at the output code it can be determined that either the P-side or the N-side is off. To pinpoint which one is off, the comparators must be used. The input voltage is also monitored using a comparator and a 4-bit digital-to-analog converter (DAC) whose levels are set by the COMP_TH[2:0] bits in the LOFF register. The output of the comparators are stored in the LOFF_STAT register. These two registers are available as a part of the output data stream. (See the Data Output Protocol (DOUT) subsection of the SPI Interface section.) If dc lead-off is not used, the lead-off comparators can be powered down by setting the PD_LOFF_COMP bit in the CONFIG2 register.

An example procedure to turn on dc lead-off is given in the Lead-Off section.