Datasheet

22 Fm
ToADCReferenceInputs
VCAP1
10 Fm
VREFP
VREFN
Bandgap
2.4Vor4V
AVSS
R1
(1)
R3
(1)
R2
(1)
100W
100kW
100W
OPA211
10pF
0.1 Fm
+5V
0.1 Fm10 Fm
100 Fm22 Fm
OUTVIN+5V
TRIM
22 Fm
REF5025
ToVREFPPin
ADS1294, ADS1294R
ADS1296, ADS1296R
ADS1298, ADS1298R
www.ti.com SBAS459I –JANUARY 2010–REVISED JANUARY 2012
REFERENCE
Figure 35 shows a simplified block diagram of the internal reference of the ADS129x. The reference voltage is
generated with respect to AVSS. When using the internal voltage reference, connect VREFN to AVSS.
(1) For V
REF
= 2.4V: R1 = 12.5kΩ, R2 = 25kΩ, and R3 = 25kΩ. For V
REF
= 4V: R1 = 10.5kΩ, R2 = 15kΩ, and R3 = 35kΩ.
Figure 35. Internal Reference
The external band-limiting capacitors determine the amount of reference noise contribution. For high-end ECG
systems, the capacitor values should be chosen such that the bandwidth is limited to less than 10Hz, so that the
reference noise does not dominate the system noise. When using a 3V analog supply, the internal reference
must be set to 2.4V. In case of a 5V analog supply, the internal reference can be set to 4V by setting the
VREF_4V bit in the CONFIG2 register.
Alternatively, the internal reference buffer can be powered down and VREFP can be applied externally. Figure 36
shows a typical external reference drive circuitry. Power-down is controlled by the PD_REFBUF bit in the
CONFIG3 register. By default the device wakes up in external reference mode.
Figure 36. External Reference Driver
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Product Folder Link(s): ADS1294 ADS1294R ADS1296 ADS1296R ADS1298 ADS1298R