18-Bit, Single-Channel, Ultra-Low-Power, Delta-
Sigma ADCs with Programmable Gain and GPIO
IREF1 = K x IREF2
Figure 8. RTD Temperature Measurement Circuit
Figure 9. Resistive Bridge Measurement Circuit
See Figure 8 for the RTD temperature measurement circuit
and Figure 9 for a resistive bridge measurement circuit.
Adding more active circuitry to the analog input signal
path is not always the best solution to a small-signal
problem. Sometimes, increasing the dynamic range of
an active device can lead to a simpler solution that also
helps power consumption and linearity.
Often, circuit designers immediately look for an exter-
nal op amp or programmable gain amplifier (PGA)
when confronted with coupling low-amplitude signals
to sampled digital systems. In many cases, choosing
an ADC with more dynamic range and better low-noise
performance yields a solution that works better, simpler,
and with less power.
One such example is measurements from a strain gauge
in a Wheatstone bridge configuration. Assuming a dif-
ferential output signal from the bridge in Figure 10, the
bridge’s output voltage varies from 5mV to 105mV,
while the noise of the bridge itself limits the sensitivity
to approximately 1FV. This gives approximately 100,000
discrete levels that are available for quantization, a feat
accomplished quite well with any ADC having 17 bits or
more of usable resolution. However, as it is not likely that
a 19-bit ADC will have an input range of 105mV, a gain
stage is needed to boost the signal to span the input
range of the ADC (typically between 3V and 5V).
This solution requires finding an amplifier and associated
passives that meet the overall system noise and linearity
needs. Also, the power consumed in the gain stage may
equal or surpass that of the ADC itself, a fact that is sig-
nificant in systems where power consumption is severely
constrained, such as portable sensors or 4–20mA loops.
The low-noise floor of the MAX11209 family of 16-/18-/20-
bit devices gives the designer the ability to use simple
binary shifting (digital gain) of the data word to align the
sample range with the available code space. Digital gain
is internally available in the MAX11209.