Datasheet

FS98O22
Rev. 1.6 117/146
12. Low Noise Operation Amplifier Guide
The input noise of CMOS OPAMP is generally much larger than the one of a Bipolar OPAMP. Moreover,
the flick noise (1/f noise) of CMOS is a killer for low frequency small signal measurement. But the need for input
bias current in Bipolar OPAMP causes that some transducers can not be used. In general, bipolar process is
not good for highly integrated Ics. FS98O22 use special CMOS low noise circuit design, and under normal
conditions, the input noise is controlled under 1μVpp (0.1Hz~1Hz). FS98O22 is good for transducer
applications because there is no need to consider input bias current.
Most of the input noise in CMOS OPAMP comes from input differential amplification. S_CHCK can be set
to switch the differential amplification: 00 for positive Offset Voltage, 01 for negative Offset voltage. When using
one clock pulse to switch input differential amplification, that is called chopper mode. In general, chopper
frequency is set between 1 kHz and 2 KHz.
Under chopper mode, the input noise peak-to-peak voltage in FS98O22 is less than 0.5μV (0.1Hz~1Hz).
But an equivalent input cur
rent of less than 100pA is generated, due to the effect of switching.
12.1. Single End Amplifier Application
Measurement of small signal usually takes consideration of the drifting of an OPAMP offset voltage. In the
Figure below, the negative input is connected to AGND. It is also possible to measure the ADC’s negative input
and deduct this value; in order to correct the error caused by the Amplifier’s offset voltage drifting. Because
AGND provides current output in applications, AIN1 is used as negative input measurement point to avoid
unnecessary voltage error.
OPAMP input offset is amplified by an amplifier then inputted to ADC. Too much amplification can cause
OPAMP output move beyond ADC linear operation range. Hence, under normal conditions, OPAMP
amplification should be less than 50 times.
Please see Figure 12-1 for example.
Figure 12-1 single end amplifier application example
FORTUNE'
Properties
For Reference Only