Application Information; Analog Input - SG Micro SGM5200 Manual

SGM5200

APPLICATION INFORMATION

In general applications, when the internal multiplexer is
updated, the previously converted channel charge is stored
in the 31pF internal input capacitance that disturbs the
voltage at the newly selected channel. This disturbance is
expected to settle to less than 1/2LSB during sampling
(acquisition) time to avoid degrading converter performance.
The initial absolute disturbance error at the channel input
must be less than 0.5V to prevent source current saturation
or slewing that causes significantly long settling times.
Fortunately, significantly reducing disturbance error is easy
to accomplish by simply placing a large enough capacitor at
the input of each channel. Specifically, with a 300pF
capacitor, instantaneous
disturbance error less than 0.47V because the internal input
capacitance can only hold up to 155pC (or 5V × 31pF). The
remaining error must be corrected by the voltage source at
each input, with impedance low enough to settle within
1/2LSB. Figure 16 shows the equivalent circuit model for
the MUX and ADC.
From sensors, INA etc.
There is a restriction on
source impedance.
R ≤ 50Ω
SOURCE
NOTE: GPIO0, GPIO1, GPIO2 and GPIO3 are available only in TSSOP packaged device. TQFN device offers GPIO0 only. As
a result, all references related to GPIO0 only are valid in case of TQFN packaged device.
SG Micro Corp
www.sg-micro.com
charge distribution
CH0
150Ω
6pF
CH15
80MΩ
6pF
Figure 16. ADC and MUX Equivalent Circuit
MXO
CH0
CH1
CH2
CH15
Figure 17. Typical Application Diagram Showing MXO Shorted to AINP
12-Bit, 1MSPS, 16 Channels,
Single-Ended, Serial Interface ADC

Analog Input

The SGM5200 offers 12-bit ADC with the 16-channel
multiplexer for analog inputs. The multiplexer output is
available on the MXO pin. The AINP pin is the ADC input
pin. The device offers flexibility for a system designer as
both signals are accessible externally.
Typically it is convenient to short MXO to the AINP pin so
that signal input to each multiplexer channel can be
processed independently. In this condition, the source
impedance of each channel is limited to 50Ω or less. Higher
source impedance may affect the signal settling time after a
multiplexer channel change. This condition can affect
keeps
linearity and total harmonic distortion. Refer to Figure 17.
MXO AINP
6pF
12pF
CH0 assumed to be on
CH15 assumed to be off
AINP
ADC
REFP
REF
SGM4029-2.5
O/P
10μF
80Ω
7pF
GPIO0, High Alarm
GPIO1, Low Alarm
GPIO2, Range
GPIO3, nPD
To Host
SDO
SDI
SCLK
nCS
REFM
SEPTEMBER 2021
30