OPA188_13 Datasheet, PDF (18/33 Pages) Texas Instruments – Precision, Low-Noise, Rail-to-Rail Output,36-V, Zero-Drift Operational Amplifiers

OPA188
SBOS642A – MARCH 2013 – REVISED MARCH 2013
www.ti.com
CAPACITIVE LOAD AND STABILITY
The device dynamic characteristics are optimized for a range of common operating conditions. The combination
of low closed-loop gain and high capacitive loads decreases the amplifier phase margin and can lead to gain
peaking or oscillations. As a result, larger capacitive loads must be isolated from the output. The simplest way to
achieve this isolation is to add a small resistor (for example, R_OUT equal to 50 Ω) in series with the output.
Figure 41 and Figure 42 illustrate graphs of small-signal overshoot versus capacitive load for several values of
R_OUT. Also, refer to the Applications Report, Feedback Plots Define Op Amp AC Performance (SBOA015),
available for download from www.ti.com, for details of analysis techniques and application circuits.
40
R_L = 10 kW
35
R_ISO = 0 W
30
R_ISO = 25 W
25
R_ISO = 50 W
20
15
G = +1
+18 V
10
Device
RISO
-18 V
RL
CL
5
0
0 100 200 300 400 500 600 700 800 900 1000
Capacitive Load (pF)
Figure 41. Small-Signal Overshoot versus
Capacitive Load (100-mV Output Step)
40
R_ISO = 0 W
35
R_ISO = 25 W
30
R_ISO = 50 W
25
20
15
10
5
R_L = R_F = 10 kW
0
0 100 200 300
400
500
G = -1 ^RI ^{= 10 kW R}F ^{= 10 kW}
+18 V
Device
-18 V
RISO
CL
600 700 800 900 1000
Capacitive Load (pF)
Figure 42. Small-Signal Overshoot versus
Capacitive Load (100-mV Output Step)
ELECTRICAL OVERSTRESS
Designers often ask questions about the capability of an operational amplifier to withstand electrical overstress.
These questions tend to focus on the device inputs, but may involve the supply voltage pins or even the output
pin. Each of these different pin functions have electrical stress limits determined by the voltage breakdown
characteristics of the particular semiconductor fabrication process and specific circuits connected to the pin.
Additionally, internal electrostatic discharge (ESD) protection is built into these circuits to protect them from
accidental ESD events both before and during product assembly.
Having a good understanding of this basic ESD circuitry and its relevance to an electrical overstress event is
helpful. See Figure 43 for an illustration of the ESD circuits contained in the OPA188 (indicated by the dashed
line area). The ESD protection circuitry involves several current-steering diodes connected from the input and
output pins and routed back to the internal power-supply lines, where the diodes meet at an absorption device
internal to the operational amplifier. This protection circuitry is intended to remain inactive during normal circuit
operation.
An ESD event produces a short-duration, high-voltage pulse that is transformed into a short-duration, high-
current pulse while discharging through a semiconductor device. The ESD protection circuits are designed to
provide a current path around the operational amplifier core to prevent damage. The energy absorbed by the
protection circuitry is then dissipated as heat.
When an ESD voltage develops across two or more amplifier device pins, current flows through one or more
steering diodes. Depending on the path that the current takes, the absorption device may activate. The
absorption device has a trigger, or threshold voltage, that is above the normal operating voltage of the OPA188
but below the device breakdown voltage level. When this threshold is exceeded, the absorption device quickly
activates and clamps the voltage across the supply rails to a safe level.
When the operational amplifier connects into a circuit (such as the one Figure 43 depicts), the ESD protection
components are intended to remain inactive and do not become involved in the application circuit operation.
However, circumstances may arise where an applied voltage exceeds the operating voltage range of a given pin.
Should this condition occur, there is a risk that some internal ESD protection circuits may be biased on, and
conduct current. Any such current flow occurs through steering-diode paths and rarely involves the absorption
device.
18
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