Rainbow Electronics MAX44252 - Manual (Page 10)

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MAX44251/MAX44252
20V, Ultra-Precision, Low-Noise Op Amps
Detailed Description
The MAX44251/MAX44252 are high-precision amplifiers
that have less than 3FV of typical input-referred offset
and low flicker noise. These characteristics are achieved
through an autozeroing technique that samples and finds
repeating patterns of signal to cancel the input offset volt-
age and 1/f noise of the amplifier.
Autozero
The ICs feature an autozero circuit that allows the devices
to achieve less than 6FV (max) of input offset voltage at
room temperature and eliminate the 1/f noise.
Noise Suppression
Flicker noise, inherent in all active devices, is inversely
proportional to frequency presented. Charges at the
oxide-silicon interface that are trapped-and-released by
MOSFET oxide occurs at low frequency more often. For
this reason, flicker noise is also called 1/f noise.
Electromagnetic interference (EMI) noise occurs at high-
er frequency that results in malfunction or degradation of
electrical equipment.
The ICs have an input EMI filter to avoid the output get-
ting affected by radio frequency interference. The EMI
filter composed of passive devices presents significant
higher impedance to higher frequency.
High Supply Voltage Range
The ICs feature 1.15mA current consumption per channel
and a voltage supply range from either 2.7V to 20V single
supply or ±1.35V to ±10V split supply.
Applications Information
The ICs are ultra-high-precision operational amplifiers with
a high supply voltage range designed for load cell, medi-
cal instrumentation and precision instrument applications.
These devices are also designed to interface with pres-
sure transducers and are ideal for precision weight scale
application as shown in Figure 1.
ADC Buffer Amplifier
The 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 pro-
duce a very small voltage change at their outputs, there-
fore 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 MAX44251/MAX44252's low input offset voltage and
low noise allow a gain circuit prior to the MAX11211 with-
out losing any dynamic range at the ADC.
Figure 1. Weight Scale Application Circuit
20V
20V
20V
3.3V
3V
V DD
MICRO
PROCESSOR
V OUT
50R G
50R G
R G
V REF V DD
V SS
V IN-
V IN+
C 1
BUFFER
BUFFER
MAX44251
MAX44251
MAX11211
MAX6126
MAX44251
R
R
20V
1.5V
BUFFER
R
R
R 1
OUTPUT