OPA1662-Q1_16 Datasheet, PDF (15/33 Pages) Texas Instruments – Operational Amplifier

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OPA1662-Q1
SLOS805C – JULY 2012 – REVISED AUGUST 2016
Feature Description (continued)
8.3.2 Input Protection
The input terminals of the OPA1662-Q1 are protected from excessive differential voltage with back-to-back
diodes, as Figure 44 illustrates. In most circuit applications, the input protection circuitry has no consequence.
However, in low-gain or G = 1 circuits, fast ramping input signals can forward bias these diodes because the
output of the amplifier cannot respond rapidly enough to the input ramp. If the input signal is fast enough to
create this forward bias condition, the input signal current must be limited to 10 mA or less. If the input signal
current is not inherently limited, an input series resistor (R_I) or a feedback resistor (R_F) can be used to limit the
signal input current. This resistor degrades the low-noise performance of the OPA1662-Q1 and is examined in
Noise Performance. Figure 44 shows an example configuration when both current-limiting input and feedback
resistors are used.
R_F
-
Input
R_I
+
OPA1662-Q1
Output
Figure 44. Pulsed Operation
8.3.3 Noise Performance
Figure 45 shows the total circuit noise for varying source impedances with the op amp in a unity-gain
configuration (no feedback resistor network, and therefore no additional noise contributions).
The OPA1662-Q1 (GBW = 22 MHz, G = 1) is shown with total circuit noise calculated. The op amp itself
contributes both a voltage noise component and a current noise component. The voltage noise is commonly
modeled as a time-varying component of the offset voltage. The current noise is similarly modeled as the time-
varying component of the input bias current and reacts with the source resistance to create a voltage component
of noise. Therefore, the lowest noise op amp for a given application depends on the source impedance. For low
source impedance, current noise is negligible, and voltage noise generally dominates. The low voltage noise of
the OPA1662-Q1 op amp makes them a better choice for low source impedances of less than 1 kΩ.
10k
1k
R_S
E_o² = e_n² + (i_nR_S)² + 4KTR_S
E_O
OPA166x
100
OPA165x
10
1
100
Resistor Noise
1k
10k
100k
Source Resistance (W)
1M
G003
The equation calculates total circuit noise, where:
• e_n is the voltage noise
• i_n is the current noise
• R_S is the source impedance
• k is Boltzmann’s constant = 1.38 × 10^–23 J/K
• T is the temperature in Kelvins (K)
Figure 45. Noise Performance of the OPA1662-Q1 in Unity-Gain Buffer Configuration
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