Datasheet

100 Fm

To ADC Reference Inputs

VCAP1

10 Fm

VREFP

VREFN

Bandgap

4.5 V

AVSS

(1)

−140

−120

−100

−80

−60

−40

−20

0 0.5 1 1.5 2 2.5 3 3.5 4

Normalized Frequency (f

MOD

)

Gain (dB)

G029

ADS1299

www.ti.com

SBAS499A –JULY 2012–REVISED AUGUST 2012

Figure 29. Transfer Function of On-Chip Decimation Filters

Until 4 f

MOD

for DR[2:0] = 000 and DR[2:0] = 110

REFERENCE

Figure 30 shows a simplified block diagram of the ADS1299 internal reference. The 4.5-V reference voltage is

generated with respect to AVSS. When using the internal voltage reference, connect VREFN to AVSS.

(1) For V

REF

= 4.5 V: R1 = 9.8 kΩ, R2 = 13.4 kΩ, and R3 = 36.85 kΩ.

Figure 30. Internal Reference

The external band-limiting capacitors determine the amount of reference noise contribution. For high-end EEG

systems, the capacitor values should be chosen such that the bandwidth is limited to less than 10 Hz so that the

reference noise does not dominate system noise.

Alternatively, the internal reference buffer can be powered down and VREFP can be applied externally. Figure 31

shows a typical external reference drive circuitry. Power-down is controlled by the PD_REFBUF bit in the

CONFIG3 register. This power-down is also used to share internal references when two devices are cascaded.

By default, the device wakes up in external reference mode.

Product Folder Link(s): ADS1299