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ManualsBrandsTEXAS INSTRUMENTS ManualsData collecting ICsData acquisition IC - Analog front end (AFE) 16 Bit TQFP 64
51525354555657585960
a)LOFF_FLIP=0 a)LOFF_FLIP=1
PGA
10MW
10MW
AVDD
INP
INN
PGA
10MW
10MW
AVDD
INP
INN
ADS119x ADS119x
a)Pull-Up/Pull-DownResistors b)CurrentSource
PGA
10MW
10MW
AVDD
INP
INN
PGA
AVDD
INP
INN
ADS119xADS119x
ADS1194, ADS1196
ADS1198
SBAS471C –APRIL 2010– REVISED NOVEMBER 2011
www.ti.com
DC Lead-Off
In this method, the lead-off excitation is with a dc signal. The dc excitation signal can be chosen from either a
pull-up/pull-down resistor or a current source/sink, shown in Figure 47. The selection is done by setting the
VLEAD_OFF_EN bit in the LOFF register. One side of the channel is pulled to supply and the other side is pulled
to ground. The pull-up resistor and pull-down resistor can be swapped (as shown in Figure 48) by setting the bits
in the LOFF_FLIP register. In case of current source/sink, the magnitude of the current can be set by using the
ILEAD_OFF[1:0] bits in the LOFF register. The current source/sink gives larger input impedance compared to the
10MΩ pull-up/pull-down resistor.
Figure 47. DC Lead-Off Excitation Options Figure 48. LOFF_FLIP Usage
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/or 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 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_STATP and LOFF_STATN registers. These two
registers are available as a part of the output data stream. (See the Data Output (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 CONFIG4 register.
An example procedure to turn on dc lead-off is given in the Lead-Off subsection of the Quick-Start Guide section.
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