Test Options on the EVM
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Figure 52. Scope Tab for Input Short Test
8.2 External Input Short with 5K Resistor
There is an option on board to tie the positive and negative input of the channel to a common voltage
(VCM) on BIAS_ELEC through 5K resistors. The following jumper settings are needed for this test. On
JP6 short pin 1 and pin 2. On JP25 short (1-2) and (2-3). The connecter J6 must have jumpers across
from left to right to connect the inputs to the ADS1299 channels. The noise from U11 which is used to
generate the BIAS_ELEC appears as common mode noise for this test and is rejected. Same is true for
noise from resistor R10 in BIAS_ELEC path. The only noise source present are two 5K resistors in the
input path and the channel noise. This test is useful to measure the effect of input bias current on noise.
The PGA in ADS1299 has CMOS input and thus has negligible current noise. The input bias current is as
a result of chopping the PGA to remove flicker noise. This bias current doesn’t manifest itself as noise and
appears like a DC offset in presence of 5K input impedance. The Channel control registers must be
programmed as shown in Figure 53. The results in the analysis tab are shown in Figure 54. The average
peak-to-peak noise for this test is 1.27µV. The increase in noise is due to the noise from 5K resistors. The
two 5K resistors contribute about 0.67µVpp in 65Hz bandwidth.
44
EEG Front-End Performance Demonstration Kit SLAU443–May 2012
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