Datasheet

ADS1293
SNAS602B FEBRUARY 2013REVISED MARCH 2013
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This AC voltage will be digitized by the AFE, and the result is available in the digital AFE output signals. The lead
connectivity can be determined in the digital domain by applying an FFT to the digital data and by measuring the
amplitude of the tone at the AC LOD excitation frequency. It should be noted that the digital AC LOD can only
determine the series connectivity of the two leads attached to the inputs of a differential channel, and hence the
connectivity of the individual input pins can only be determined by the DC or the analog AC LOD.
Figure 26. Simplified Digital Analog AC Lead-off Detect Block Diagram
Figure 26 shows a simplified block diagram of the digital AC LOD. Follow the procedures below to activate the
Digital AC LOD:
1. Select the Digital AC lead-off mode by setting bit SELAC_LOD = 1 and ACAD_LOD = 0 in the LOD_CN
register.
2. Program the excitation frequency Φ by using the ACDIV_LOD and ACDIV_FACTOR bits in the LOD_AC_CN
register. See equation in Analog AC Lead-Off Detect
3. Enable which channel the digital AC LOD will be applied to by selecting the EN_LOD[2:0] bits in the
LOD_EN register . These bits correspond to the AFE channels CH3 to CH1 from MSB to LSB, respectively.
4. Determine the phase of the injected current to the AFE channels by programming the EN_LOD[5:3] bits in
the LOD_EN register.
The EN_LOD[5:3] bits determine the phase of the injected current to the AFE channels CH3 to CH1 from MSB to
LSB, respectively. A bit set to 1 means that the corresponding channel will receive an anti-phase excitation
current in respect to the frequency divider's phase. In some applications, it may be necessary to invert the sign of
the digital AC lead-off test current on a channel. Consider an example where the first AFE is configured through
the flexible routing switch to measure the voltage between IN1 and IN2, and the second AFE is configured
through the flexible routing switch to measure the voltage between IN2 and IN3.
In this configuration, if digital AC LOD test currents are applied to the inputs of both AFEs, the test current that is
applied to the negative input of the first AFE and the test current that is applied to the positive input of the
second AFE are both flowing through IN2. Depending on the sign of the test current in the second AFE, these
currents can add up or cancel each other. If the currents add up, the system will correctly measure the
differential input impedance on both AFE channels. If the currents on IN2 cancel, the test current will only flow
through IN1 and IN3, and the impedance of the electrode connected to IN2 cannot be measured. To apply the
digital AC LOD to CH3 and CH2, set EN_LOD[2] = 1 and EN_LOD[1] = 1. Then, by programming EN_LOD[5] = 0
and EN_LOD[4] = 1, CH3 and CH2 will receive excitation currents in-phase and anti-phase, respectively.
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