OPA2320-Q1_15 Datasheet, PDF (19/31 Pages) Texas Instruments – Precision, 20-MHz, 0.9-pA, Low-Noise, RRIO,CMOS Operational Amplifier

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160
140
120
100
80
60
40
20
0
-20
1
10 100
V_S = ±2.5 V
1k 10k 100k 1M
Frequency (Hz)
C_(L) = 50 pF
0
Phase
Gain -20
-40
-60
-80
-100
-120
-140
-160
-180
10M 100M
OPA2320-Q1
SLOS884 – SEPTEMBER 2014
1000
100
10
1
10
100
1k
10k
100k
1M
V_S = 1.8 to 5.5 V
Frequency (Hz)
Figure 38. Open-Loop Gain and Phase vs Frequency
Figure 39. Input Voltage Noise Spectral Density vs
Frequency
8.2.2 High-Impedance Sensor Interface
Many sensors have high source impedances that may range up to 10 MΩ, or even higher. The output signal of
sensors often must be amplified or otherwise conditioned by means of an amplifier. The input bias current of this
amplifier can load the sensor output and cause a voltage drop across the source resistance, as shown in
Figure 40, where (V_(+INx) = V_S – I_(BIAS) × R_(S)). The last term, I_(BIAS) × R_(S), shows the voltage drop across R_(S). To
prevent errors introduced to the system as a result of this voltage, an op amp with very low input bias current
must be used with high impedance sensors. This low current keeps the error contribution by I_(BIAS) × R_(S) less
than the input voltage noise of the amplifier, so that it does not become the dominant noise factor. The
OPA2320-Q1 device series of op amps feature very low input bias current (typically 200 fA), and are therefore
ideal choices for such applications.
R_(S)
100 kΩ
I_{IB V(+INx)}
V_(V+)
OPA320-Q1
V_(V–)
V_O
R_(F)
R_(G)
Figure 40. Noise as a Result of I_(BIAS)
8.2.3 Driving ADCs
The OPA2320-Q1 device series op amps are well-suited for driving sampling analog-to-digital converters (ADCs)
with sampling speeds up to 1 MSPS. The zero-crossover distortion input stage topology allows the OPA2320-Q1
device to drive ADCs without degradation of differential linearity and THD.
The OPA2320-Q1 device can be used to buffer the ADC switched input capacitance and resulting charge
injection while providing signal gain. Figure 42 shows the OPA2320-Q1 device configured to drive the ADS8326.
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