OPA365-Q1_15 Datasheet, PDF (16/26 Pages) Texas Instruments – 50-MHz Low-Distortion High-CMRR Rail-to-Rail I/O, Single-Supply Operational Amplifier

OPA365-Q1
OPA2365-Q1
SBOS512C – MARCH 2010 – REVISED APRIL 2012
www.ti.com
An RC network, consisting of R1 and C1, is included between the op amp and the ADS8361. It not only provides
a high-frequency filter function, but more importantly serves as a charge reservoir used for charging the
converter internal hold capacitance. This capability assures that the op amp output linearity is maintained as the
ADC input characteristics change throughout the conversion cycle. Depending on the particular application and
ADC, some optimization of the R1 and C1 values may be required for best transient performance.
Figure 13 illustrates the OPA2365 dual op amp providing signal conditioning within an ADS1258 bridge sensor
circuit. It follows the ADS1258 16:1 multiplexer and is connected as a differential in/differential out amplifier. The
voltage gain for this stage is approximately 10V/V. Driving the ADS1258 internal ADC in differential mode, rather
than in a single-ended, exploits the full linearity performance capability of the converter. For best common-mode
rejection the two R2 resistors should be closely matched.
+5V
RFI
0.1µF
10µF
2kΩ
RFI
AIN0
AVSS
AVDD
2kΩ
RFI
AIN1
2kΩ
RFI
AIN14
ADS1258
REFP
+ _10µF
REFN
0.1µF
2kΩ
RFI
RFI
AIN15
AINCOM
+5V
0.1µF
OPA2365
R₂ = 10 kΩ
2.2nF
R₃
47Ω
R₁ = 2.2kΩ
R₂ = 10 kΩ
OPA2365
R₃
47Ω
Figure 13. Conditioning Input Signals to the ADS1258 on a Single-Supply
Note that in Figure 13, the amplifiers, bridges, ADS1258, and internal reference are powered by the same single
+5V supply. This ratiometric connection helps cancel excitation voltage drift effects and noise. For best
performance, the +5V supply should be as free as possible of noise and transients.
When the ADS1258 data rate is set to maximum and the chop feature enabled, this circuit yields 12 bits of noise-
free resolution with a 50mV full-scale input.
The chop feature is used to reduce the ADS1258 offset and offset drift to very low levels. A 2.2 nF capacitor is
required across the ADC inputs to bypass the sampling currents. The 47Ω resistors provide isolation for the
OPA2365 outputs from the relatively large, 2.2 nF capacitive load. For more information regarding the ADS1258,
see the product data sheet available for download at www.ti.com.
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