LTC6802-2
31
68022fa
Figure 18. Noise Filtering of the LTC6802-2 ADC
FILTER GAIN (dB)
FREQUENCY (Hz)
100k10
10
–60
100 1k 10k
0
–10
–20
–30
–40
–50
68022 F18
Converter Details
The LTC6802-2’s ADC has a second-order delta-sigma
modulator followed by a Sinc2, finite impulse response
(FIR) digital filter. The front-end sample rate is 512ksps,
which greatly reduces input filtering requirements. A
simple 16kHz, 1-pole filter composed of a 100Ω resistor
and a 0.1μF capacitor at each input will provide adequate
filtering for most applications. These component values
will not degrade the DC accuracy of the ADC.
Each conversion consists of two phases—an autozero
phase and a measurement phase. The ADC is autozeroed
at each conversion, greatly improving CMRR. The second
half of the conversion is the actual measurement.
Noise Rejection
Figure 18 shows the frequency response of the ADC. The
roll-off follows a Sinc2 response, with the first notch at
4kHz. Also shown is the response of a 1-pole, 850Hz filter
(187μs time constant) which has the same integrated
response to wideband noise as the LTC6802-2’s ADC,
which is about 1350Hz. This means that if wideband noise
applicaTions inForMaTion
is applied to the LTC6802-2 input, the increase in noise
seen at the digital output will be the same as an ADC with
a wide bandwidth (such as a SAR) preceded by a perfect
1350Hz brickwall lowpass filter.
Thus if an analog filter is placed in front of a SAR converter
to achieve the same noise rejection as the LTC6802-2 ADC,
the SAR will have a slower response to input signals. For
example, a step input applied to the input of the 850Hz filter
will take 1.55ms to settle to 12 bits of precision, while the
LTC6802-2 ADC settles in a single 1ms conversion cycle.
This also means that very high sample rates do not provide
any additional information because the analog filter limits
the frequency response.
While higher order active filters may provide some im-
provement, their complexity makes them impractical for
high-channel count measurements as a single filter would
be required for each input.
Also note that the Sinc2 response has a 2nd order roll-off
envelope, providing an additional benefit over a single-pole
analog filter.
