Datasheet

200
180
160
140
120
100
80
60
40
20
0
0.3
2.8
InputCommon-ModeVoltage(V)
WCTInputLeakageCurrent(pA)
0.8
T =+25 C°
A
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
80
60
40
20
0
0.3
2.8
InputCommon-ModeVoltage(V)
WCTInputLeakageCurrent(pA)
0.8
T =+25 C°
A
LPMode
HRMode
1.3 1.8 2.3
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
SBAS459I –JANUARY 2010–REVISED JANUARY 2012 www.ti.com
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, f
MOD
/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 5mV
PP
.
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 f
MOD
/16 f
MOD
/16
001 f
MOD
/32 f
MOD
/16
010 f
MOD
/64 f
MOD
/16
011 f
MOD
/128 f
MOD
/16
100 f
MOD
/128 f
MOD
/16
101 f
MOD
/128 f
MOD
/16
110 f
MOD
/128 f
MOD
/16
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