ADS1294CZXGR datasheet(40/114 Pages) TI2

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a)Pull-Up/Pull-DownResistors

b)CurrentSource

PGA

10MW

AVDD

INP

INN

PGA

AVDD

INP

INN

ADS129x

a)LOFF_FLIP=0

a)LOFF_FLIP=1

PGA

10MW

AVDD

INP

INN

PGA

10MW

AVDD

INP

INN

ADS129x

ADS1294, ADS1294R, ADS1296, ADS1296R, ADS1298, ADS1298R

SBAS459K – JANUARY 2010 – REVISED AUGUST 2015

www.ti.com

9.3.1.7.4.1

DC Lead-Off

In this approach, the lead-off excitation is accomplished with a dc signal. Choose a dc excitation signal from

either a pullup or pulldown resistor, or from a current source or sink system, as shown in Figure 40. Select 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. Swap the pullup resistor and pulldown resistor by setting the bits in the LOFF_FLIP

ILEAD_OFF[1:0] bits in the LOFF register. The current source or sink gives larger input impedance compared to

the 10-M

Ω pullup or pulldown resistor.

Figure 41. LOFF_FLIP Usage

Figure 40. DC Lead-Off Excitation Options

Response sensing is achieved either by looking at the digital output code from the device, or by monitoring the

input voltages with on-chip comparators. If either of the electrodes is off, the pullup or pulldown resistors saturate

the channel. Look at the output code to determine if either the P-side or the N-side is off. To pinpoint which side

is off, check the comparator outputs. During conversion, the input voltage is simultaneously monitored by using a

comparator and a 4-bit DAC with levels that are set by the COMP_TH[2:0] bits in the LOFF register. The

comparator outputs 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 Pin (DOUT) 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 section.

9.3.1.7.4.2

AC Lead-Off

This method uses an out-of-band ac signal for excitation. The ac signal is generated by providing pullup and

pulldown resistors at the input with a fixed frequency. The ac signal is passed through an antialiasing filter to

prevent aliasing. Select the frequency with the FLEAD_OFF[1:0] bits in the LOFF register. The excitation

frequency is a function of the output data rate and is fDR / 4. This out-of-band excitation signal is passed through

the channel and measured at the output.

AC signal sensing is achieved by passing the signal through the channel to digitize the signal, and measuring the

output. The ac excitation signals are introduced at a frequency that is above the band of interest, generating an

out-of-band differential signal that can be filtered out separately and processed. By measuring the magnitude of

the excitation signal at the output spectrum, the lead-off status is calculated. Therefore, the ac lead-off detection

is accomplished simultaneously with the ECG signal acquisition.

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ADS1294CZXGR Datasheet(PDF) 40 Page - Texas Instruments

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