ADS1294 datasheet(61/88 Pages) TI

61 / 88 page

200

180

160

140

120

100

0.3

2.8

InputCommon-ModeVoltage(V)

0.8

T =+25 C

1.3

1.8

2.3

DR=0.5kSPS,0.25kSPS

DR=1kSPS

DR=2kSPS

DR=4kSPS

DR=8kSPS

DR=16kSPS

DR=32kSPS

200

180

160

140

120

100

0.3

2.8

InputCommon-ModeVoltage(V)

0.8

T =+25 C

LPMode

HRMode

1.3

1.8

2.3

ADS1294, ADS1294R

ADS1296, ADS1296R

ADS1298, ADS1298R

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SBAS459H

– JANUARY 2010 – REVISED OCTOBER 2011

As mentioned previously in this section, all three WCT amplifiers can be connected to one of eight analog input

pins. The inputs of the amplifiers are chopped and the chopping frequency varies with the data rates of the

ADS129x. The chop frequency for the three highest data rates scale 1:1. For example, at 32kSPS data rate, the

chop frequency is 32kHz in HR mode with WCT_CHOP = 0. The chopping frequency of the four lower data rates

is fixed to 4kHz. When WCT_CHOP = 1, the chop frequency is fixed to highest data rate (that is, fMOD/16)

frequency, as shown in Table 13. The chop frequency shows itself at the output of the WCT amplifiers as a small

square wave riding on dc. The amplitude of the square wave is the offset of the amplifier and is typically 5mVPP.

This artifact as a result of chopping is out-of-band and thus does not interfere with ECG-related measurements.

As a result of the chopping function, the input current leakage on the pins with WCT amplifiers connected sees

increased leakage currents at higher data rates and as the input common voltage swings closer to 0V (AVSS),

as described in Figure 54.

Note that if the output of a channel connected to the WCT amplifier (for example, the V lead channels) is

connected to one of the PACE amplifiers for external PACE detection, the artifact of chopping appears at the

PACE amplifier output.

Figure 54. WCT Input Leakage Current versus

Figure 55. WCT Input Leakage Current versus

Input Voltage (WCT_CHOP = 0)

Input Voltage (WCT_CHOP = 1)

Table 13. WCT Amplifiers Chop Frequency

CONFIG1.DR[2:0] BIT

CONFIG2.WCT_CHOP = 0

CONFIG2.WCT_CHOP = 1

000

fMOD/16

001

fMOD/32

fMOD/16

010

fMOD/64

fMOD/16

011

fMOD/128

fMOD/16

100

fMOD/128

fMOD/16

101

fMOD/128

fMOD/16

110

fMOD/128

fMOD/16

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