SBAA583 Application note

4.8 Improving SNR by Summing the Output of 2 ADC Channels

By using P0_R107_D (3:2), we can average the digitized data of channel 1 and 2 and make the average the digitized output of the IC.

Table 4-2 Channel Summing Mode Programmable Settings

P0_R107_D[3:2] : CH_SUM[1:0]	CHANNEL SUMMING MODE FOR INPUT CHANNELS	SNR AND DYNAMIC RANGE BOOST
00 (default)	Channel summing mode is disabled	Not applicable
01	Output channel 1 = (input channel 1 + input channel 2) / 2	Around 3-dB boost in SNR and dynamic range
01	Output channel 2 = (input channel 1 + input channel 2) / 2	Around 3-dB boost in SNR and dynamic range
10	Reserved (do not use this setting)	Not applicable
11	Reserved (do not use this setting)	Not applicable

By giving the same analog input voltage to both ADC inputs on the chip, we can realize a 3 db SNR Boost.

Equation 45.

S N R = F u l l S c a l e V a l u e \frac{(V)}{N o i s e (V_{n})}

Equation 46.

S N R = \frac{V}{V_{n}}

On summing two digital outputs:

Equation 47.

S N R_{A} = \frac{2 V}{\sqrt{{V n}^{2} + {V n}^{2}}}

Equation 48.

S N R_{A} = 1.414 \times S N R

Equation 49.

\frac{S N R_{A}}{S N R} = 3 d b

A 3 db boost in SNR is achieved.

Figure 4-15 Summing Circuit to Improve SNR

Shorting two inputs reduces the input impedance by half. For a Zin of 2.5 K, the effective Zin becomes:

Equation 50.

\frac{2.5 K}{2} = 1.25 K

Equation 51.

C = \frac{1}{2 \times 3.14 \times {F M}_{i n} \times Z_{i n}}

Equation 52.

C > \frac{1}{2 \times 3.14 \times 20 \times 1250} C > 6.3 μ f