Datasheet

11Maxim Integrated
MAX44250/MAX44251/MAX44252
20V, Ultra-Precision, Low-Noise Op Amps
ADC Buffer Amplifier
The MAX44250/MAX44251/MAX44252's low input offset
voltage, low noise, and fast settling time make these
amplifiers ideal for ADC buffers. Weigh scales are one
application that often require a low-noise, high-voltage
amplifier in front of an ADC. Figure 1 details an example
of a load cell and amplifier driven from the same Q10V
supplies, along with the MAX11211 18-bit delta sigma
ADC. Load cells produce a very small voltage change at
their outputs, therefore driving the excitation source with
a higher voltage produces a wider dynamic range that
can be measured at the ADC inputs.
The MAX11211 ADC operates from a single 2.7V to 3.6V
analog supply, offers 18-bit noise-free resolution and
0.86mW power dissipation. The MAX11211 also offers
> 100dB rejection at 50Hz and 60Hz. This ADC is part of
a family of 16-, 18-, 20-, and 24-bit delta sigma ADCs with
high precision and < 1mW power dissipation.
The MAX44250/MAX44251/MAX44252's low input offset
voltage and low noise allow a gain circuit prior to the
MAX11211 without losing any dynamic range at the ADC.
Error Budget Example
When using the ICs as an ADC buffer in strain gauge
application, the temperature drift should be taken into
consideration to determine maximum input signal. A
typical strain gauge has sensitivity specification of just
2mV/V at rated out load. This means that when the strain
gauge load cell is powered with 10V, the full-scale output
voltage is 20mV. In this application, both offset voltage
and drift are critical parameters that directly affect the
accuracy of measurement. Even though offset voltage
could be calibrated out, its drift over temperature is still
a problem.
The ICs, with a typical offset drift of 5nV/°C, guarantee
that the drift over a 10°C range is only 50nV. Setting this
equal to 0.5 LSB in a 18-bit system yields a full-scale
range of 13mV. With a single 10V supply, an acceptable
closed-loop gain of 770V/V provides sufficient gain while
maintaining headroom.
Precision Low-Side Current Sensing
The ICs’ autozero feature produces ultra-low offset
voltage and drift, making them ideal for precision cur-
rent-sensing applications. Figure 2 shows the ICs in
a low-side current-sense configuration. This circuit pro-
duces an accurate output voltage, V
OUT
equal to I
LOAD
x R
SENSE
x (1 + R
2
/R
1
).
Figure 2. Low-Side Current Sensing
V
SUPPLY
OUT
I
LOAD
R
SENSE
R
1
R
2
MAX44251
MAX44252