OPA689M Datasheet, PDF (20/22 Pages) Texas Instruments – GAIN +4 STABLE WIDEBAND VOLTAGE-LIMITING AMPLIFIER

OPA689M
GAIN +4 STABLE WIDEBAND
VOLTAGE-LIMITING AMPLIFIER
SGLS146B – MARCH 2003 – REVISED DECEMBER 2006
www.ti.com
In general, capacitive loads should be minimized for optimum high-frequency performance. The capacitance of
coax cable (29 pF/foot for RG-58) will not load the amplifier when the coaxial cable, or transmission line, is
terminated in its characteristic impedance.
Frequency Response Compensation
The OPA689 is internally compensated to be unity-gain stable at a gain of +4, and has a nominal phase margin
of 60 degrees at a gain of +6. Phase margin and peaking improve at higher gains. Recall that an inverting gain
of –5 is equivalent to a gain of +6 for bandwidth purposes (i.e., noise gain = 6).
Standard external compensation techniques work with this device. For example, in the inverting configuration,
the bandwidth may be limited without modifying the inverting gain by placing a series RC network to ground on
the inverting node. This has the effect of increasing the noise gain at high frequencies, which limits the
bandwidth.
To maintain a wide bandwidth at high gains, cascade several op amps.
In applications where a large feedback resistor is required, such as photodiode transimpedance amplifier, the
parasitic capacitance from the inverting input to ground causes peaking or oscillations. To compensate for this
effect, connect a small capacitor in parallel with the feedback resistor. The bandwidth will be limited by the pole
that the feedback resistor and this capacitor create. In other high gain applications, use a three resistor "Tee"
network to reduce the RC time constants set by the parasitic capacitances. Be careful to not increase the noise
generated by this feedback network too much.
Pulse Settling Time
The OPA689 is capable of an extremely fast settling time in response to a pulse input. Frequency response
flatness and phase linearity are needed to obtain the best settling times. For capacitive loads, such as an ADC,
use the recommended R_S in the typical performance curve "R_S vs Capacitive Load". Extremely fine-scale settling
(0.01%) requires close attention to ground return current in the supply decoupling capacitors.
The pulse settling characteristics when recovering from overdrive are very good.
Distortion
The OPA689's distortion performance is specified for a 500-Ω load, such as an ADC. Driving loads with smaller
resistance will increase the distortion as shown in Figure 40. Remember to include the feedback network in the
load resistance calculations.
−40
−45
−50
−55
−60
−65
−70
−75
−80
−85
−90
50
HARMONIC DISTORTION vs LOAD RESISTANCE
HD2
V _O = 2 Vp-p
f ₁ = 5MHz
HD3
100
Load Resistance ( Ω )
1000
Figure 40. 5-MHz Harmonic Distortion vs Load Resistance
20
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