MAX11410 Datasheet, PDF (27/95 Pages) Maxim Integrated Products – 24-Bit Multi-Channel Low-Power

MAX11410
24-Bit Multi-Channel Low-Power
1.9ksps Delta-Sigma ADC with PGA
Circuit Settling Time
The input to the ADC will require some time to settle
after changing the state of the multiplexer, PGA, current
sources, and other analog components. When using the
sequencer, insert appropriate wait times when changing
the state of any of these components.
Input Multiplexer
Settling time for changes to the state of the input multiplexer
depends on several factors. These include the delay time
of the nonoverlap circuits and the on-resistance of the
multiplexer switches, but are dominated by the output
impedance of the external source, the impedance (cables,
protection components, etc.) between the external source
and the multiplexer, any input filter capacitance, the 10pF
capacitance on the input to the PGA and modulator blocks,
and whether or not the I_DAC current sources or the V_BIAS
source are being used. To obtain an accurate conversion,
wait until the multiplexer is fully settled before starting a
new conversion. With no added capacitance at the inputs,
the settling time after a multiplexer channel change with a
2kΩ source is typically 2µs.
PGA
PGA settling time is primarily limited by the external PGA
filter. A 100nF external capacitor across CAPP and CAPN
reduces noise by limiting the bandwidth of the PGA. This
results in a 2kHz single-pole lowpass filter at the PGA's
output. Settling to 22-bit accuracy (0.25ppm) requires
15.25 time constants or 7.6mS for a 2kHz bandwidth.
Therefore, the PGA typically dominates the settling time
of the input when changing multiplexer settings or changing
the PGA's gain.
Reference Multiplexer
Settling time for the reference input multiplexer is similar
to that of the input multiplexer but with less complexity,
as the reference multiplexer has fewer channels and
does not have the I_DAC current sources or the V_BIAS
source as possible inputs. The delay is still dependent on
the on-resistance of the reference multiplexer switches,
the impedance between the reference source and the
reference multiplexer, the output impedance of the
reference source, and the input capacitance of the modu-
lator. For accurate conversions, it is important to wait until
the reference multiplexer is fully settled before starting a
new conversion.
Normally the reference should be located close to the
reference inputs, so the resistance between the source
and the input should be negligible. If the reference
source is an active voltage reference, the source impedance
should be low enough to ignore. In some cases, the
reference source may be a resistor with a value of a
few kilohms. So long as the source resistance is less
than around 10kΩ, the settling time contribution from the
reference source resistance will be less than 1µs and can
generally be ignored.
Excitation Current Source
Enabling/disabling the current source(s) will require time
for any input capacitance to charge or discharge. This can
be especially important when external capacitors have
been added at the inputs for noise filtering.
V_BIAS Source
The V_BIAS source generates a bias voltage equal to
V_DD/2. There are three V_BIAS modes, controlled by the
V_BIAS register field.
The first mode is an active bias generator featuring a
class AB output stage with a series 125kΩ resistor to
create a nominal output impedance of 125kΩ. The active
bias generator mode reduces current and channel to
channel crosstalk. In active mode, if the output is not
settled to V_DD/2, the series resistor is bypassed by a
separate low-impedance class AB output stage to
decrease settling time. When the output settles to V_DD/2,
the resistor is reasserted for improved noise filtering.
The second and third modes create the V_BIAS with resistive
voltage-dividers to offer fixed output impedance (either
125kΩ or 20kΩ) at the expense of increased current
consumption. The 125kΩ mode offers increased supply
noise filtering at the expense of increased settling time.
The 20kΩ mode offers reduced settling time, but is higher
in current consumption and offers less supply noise filtering.
The bias voltage can be switched into the input channels
via the V_{BIAS_SEL} register field.
Sensor Excitation Current Sources
The Matched Current Sources can be programmed to
provide 16 different levels of matched currents from
10μA to 1600μA with ±10% accuracy, 0.1% matching,
and 50ppm/°C temperature drift from -40°C to +85°C.
Either current source or both may be enabled, and each
current source may be connected to any one of the ten
analog inputs. Note that only one current source may be
connected to any input, and a current source may not be
connected to an input that has V_BIAS connected to it.
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