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MAXREFDES106# Health Sensor Platform 4.0
Voltage Mode
Amplitude (RMS)
Adjust current and voltage amplitudes from the ‘Current DAC’.
There are four settings, 35.4mV, 70.7mV, 177mV, and 354mV.
When Voltage is selected for Current DAC Drive Mode, the
Voltage Mode Amplitude sets voltage RMS amplitude at
DRVR. When Current is selected for Current DAC Drive
Mode, the Voltage Mode Amplitude and Current Drive Range
Resistor set the current magnitude and change the voltage
amplitude at DRVR depending on the current drive range
resistor used. Note that certain amplitudes are not available for
certain stimulus frequencies to align with safety standards.
Refer to the MAX86178 data sheet ‘BioZ Transmit Channel’
section for recommended settings and safety limits.
The Bioz drive channel is driven to the voltage selected in the
Common-Mode Voltage setting. Selecting RLD enables the
DRVN path to function as right led drive during BioZ
measurement.
When sine-wave current is used for stimulation, an external
capacitor must be inserted between DRVXC and DRVSJ pins
on the MAX86178 to couple AC-stimulus current and prevent
DC current from passing into the patient. Deselect to bypass
this external capacitor.
The BioZ channel drive channel contains a 10MΩ feedback
resistance to the current drive amplifier to maintain DC bias of
the drive electrodes during a lead-off event and reduce amplifier
settling time when the lead is reconnected. Enabling the DC
Restore applies this feedback resistance. This should be
enabled if ‘Cext’ is used.
Current DAC
Frequency Selection
The BioZ drive frequency is generated by dividing the output of
the PLL mux by K Divider and DAC OSR. Selecting respiration
mode additionally divides this frequency by the F Divider.
Sets the gain bandwidth of the voltage drive and current drive
amplifiers. Higher bandwidth is recommended for high-
frequency applications including bioimpedance analysis and
impedance cardiography. Low bandwidth is recommended for
low-frequency applications including galvanic skin response to
reduce power consumption.
Sets the output stage option for the voltage drive (1
st
) and
current drive (2
nd
) amplifiers. Higher amplifier ranges are
recommended for higher output current loading, although this
consumes more power.
Current DAC
Override
Modes
The BioZ drive channel contains settings to override the default
‘Current DAC’ configuration and drive DC current instead of
sine-wave current. Selecting DC Mode enables this
functionality. When DC Mode is selected, the DC current
magnitude is defined in ‘DC Code’ (upper 7-bits)
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