ADS1294CZXGR datasheet(94/114 Pages) TI2

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ADS1294, ADS1294R, ADS1296, ADS1296R, ADS1298, ADS1298R

SBAS459K – JANUARY 2010 – REVISED AUGUST 2015

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10.2.2.2 Detailed Design Procedure

The pace amplifiers on the ADS129x provides differential to single-ended conversion and amplification of 0.4 V/V

to whatever voltage appears at the output of the PGA of the channel from which the pace amplifier is routed.

Selecting which channels are routed to the pace amplifiers is performed in the pace detect register of the

ADS129x. The voltage that appears at the output of the pace amplifier is to be taken with respect to analog

midsupply.

Before the signal is converted by the ADS7042, the signal must be buffered by a high-speed op amp because

the inputs of the ADS7042 represent a switch-capacitor type load. The OPA320 is ideal to perform this function

because of the low input bias current and 20-MHz unity gain bandwidth. The op amp also provides the flexibility

to provide an extra gain stage before the SAR ADC, isolate filter stages, or to provide simple buffering. The

purpose of C1 and R1 are to provide ac coupling to the pacemaker detection signal. This coupling may be

necessary because electrode offset and the pacemaker pulse can both be, in some cases, up to a few hundred

millivolts.

An actively-driven signal ground is required to set the dc bias of the op amp at midsupply. It is possible to use

the voltage provided at VCAP2 on the ADS129x as a buffered midsupply voltage. The voltage at the VCAP2 pin

may be noisy, but using it to drive the common-mode voltage for both inverting and noninverting inputs to the op

amp causes the op amp to cancel that noise significantly because it is common to both inputs.

Op amp feedback resistors R4 and R5 set the gain for the OPA320. The transfer function for this configuration is

that of the noninverting op amp configuration shown in Equation 12.

(12)

Resistors R4 and R5 are chosen to set the desired gain. The series combination is approximately 100 kΩ, so that

both the feedback current is limited to within the ADS129x VCAP2 internal regulator drive strength, and the

Johnson-Nyquist noise of the resistors remains negligible.

If the OPA320 is to be used only as a buffer, remove R4 removed to provide unity gain. If ac coupling is not

desired, for best performance, replace C1 with a 0-Ω resistor and depopulate R1.

The RC network of R2, C2, R3, C3, R6, C4, R7, and C5 form isolated two-pole RC antialiasing filters for the SAR

ADC. The component values of the filter are set to provide significant attenuation at the ADC sampling

frequency, but still provide enough bandwidth to detect a pacemaker pulse. A bandwidth of greater than 2 kHz is

enough to capture a narrow 0.5-ms pacemaker pulse.

In a real-time system, data must be collected and analyzed for a pacemaker with each incoming sample. Digitally

filter data that are collected from the ADS7042 to remove out-of-band noise. Unlike a delta-sigma converter, a

SAR converter does not apply a filter to the data before it is sent to the host. There are a number of factors that

drive a decision on digital-filter implementation. Some of those factors include steepness of the response, phase

linearity, and the number of taps. When using this topology with an ADS129xR device simultaneously with the

respiration measurement circuitry, take special care to remove noise generated by the respiration modulation

circuitry.

The key to detecting a pacemaker pulse is the detection of a steep transition in the input voltage. To measure

the magnitude of the transitions in input voltages, apply a digital differentiator algorithm. The algorithm measures

the change in voltage magnitude over the span of a few samples and compares the change to a threshold

required to trigger detection. The following pseudocode exemplifies some of the processing steps required to use

this topology:

newDataPoint = collectFromADS7042( );

// Collect data from the ADS7042

// Apply combined low-pass filter and differentiator

inputRateOfChange = LPFandDifferentiator( newDataPoint );

if( abs( inputRateOfChange ) > thresholdValue )

// Check if a quick edge occurred

{

pacemakerFlag = true;

// Edge detected

}

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Product Folder Links: ADS1294 ADS1294R ADS1296 ADS1296R ADS1298 ADS1298R

ADS1294CZXGR Datasheet(PDF) 94 Page - Texas Instruments

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