Test Options on the EVM
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8.4 Noise with Buffered Common Reference Input
Connecting all the negative inputs to one reference electrode can lead to excessive leakage current on the
electrode. The typical leakage current on ADS1299 channel is 200pA. So for a 16 channel system total
leakage may be as large as 3.2nA. This number will become progressively worse as channel count is
increased. If the leakage number is not acceptable there is an option to buffer the common reference input
before connected it to all the negative inputs of the channel. On JP81 jumpers (3-4) and (5-6) are
required. On JP8 a jumper (2-3) is required and on JP7 a jumper (1-2) is needed. The GUI settings are
same as in Figure 56. Figure 57 shows a snapshot of the noise with SRB1 driven by a buffered reference.
The drawback of using the buffer in the SRB1 path is increased noise. The noise contributors in these
settings are two 5k resistors, op amp U4 and ADS1299 channel. As can be seen from the Figure 57 the
noise with this approach is larger than noise in previous three approaches. At present OPA376 is installed
on board for U4. A lower noise op amp can be used if needed.
Figure 57. Noise with OPA376 in SRB1 Path
48
EEG Front-End Performance Demonstration Kit SLAU443–May 2012
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