FREQUENCY RESPONSE COMPENSATION
The percentage change in closed-loop gain over a specified
change in output voltage level is defined as dG. dP is defined
as the change in degrees of the closed-loop phase over the
same output voltage change. dG and dP are both specified at
the NTSC sub-carrier frequency of 3.58MHz. dG and dP
increase closed-loop gain and output voltage transition. All
measurements were performed using a Tektronix model
VM700 Video Measurement Set.
Each channel of the OPA2650 is internally compensated to
be stable at unity gain with a nominal 60° phase margin.
This lends itself well to wideband integrator and buffer
applications. Phase margin and frequency response flatness
will improve at higher gains. Recall that an inverting gain of
–1 is equivalent to a gain of +2 for bandwidth purposes, i.e.,
noise gain = 2. The external compensation techniques devel-
oped for voltage feedback op amps can be applied to this
device. For example, in the non-inverting configuration,
placing a capacitor across the feedback resistor will reduce
the gain to +1 starting at f = (1/2πRFCF). Alternatively, in the
inverting configuration, the bandwidth may be limited with-
out 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, thereby
limiting the bandwidth for the inverting input signal through
the gain-bandwidth product.
DISTORTION
The OPA2650’s harmonic distortion characteristics into a
100Ω load are shown versus frequency and power output in
the typical performance curves. Distortion can be signifi-
cantly improved by increasing the load resistance as illus-
trated in Figure 5. Remember to include the contribution of
the feedback resistance when calculating the effective load
resistance seen by the amplifier.
At higher gains, the gain-bandwidth of this voltage feedback
topology will limit bandwidth according to the open-loop
frequency response curve. For applications requiring a wider
bandwidth at higher gains, consider the dual current feed-
back model, OPA2658. In applications where a large feed-
back resistor is required (such as photodiode transimpedance
circuits), precautions must be taken to avoid gain peaking
due to the pole formed by the feedback resistor and the
capacitance on the inverting input. This pole can be compen-
sated by connecting a small capacitor in parallel with the
feedback resistor, creating a cancelling zero term. In other
high-gain applications, use of a three-resistor “T” connec-
tion will reduce the feedback network impedance which
–60
(G = +1, fO = 5MHz)
–70
2fO
–80
3fO
–90
10
20
50
100
200
500
1k
reacts with the parasitic capacitance at the summing node.
Load Resistance (Ω)
FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.
PULSE SETTLING TIME
High speed amplifiers like the OPA2650 are capable of
extremely fast settling time with a pulse input. Excellent
frequency response flatness and phase linearity are required
to get the best settling times. As shown in the specifications
table, settling time for a 2V step at a gain of +1 for the
OPA2650 is extremely fast. The specification is defined as
the time required, after the input transition, for the output to
settle within a specified error band around its final value. For
a 2V step, 1% settling corresponds to an error band of
±20mV, 0.1% to an error band of ±2mV, and 0.01% to an
error band of ±0.2mV. For the best settling times, particu-
larly into an ADC capacitive load, little or no peaking in the
frequency response can be allowed. Using the recommended
RISO for capacitive loads will limit this peaking and reduce
the settling times. Fast, extremely fine scale settling (0.01%)
requires close attention to ground return currents in the
supply decoupling capacitors. For highest performance, con-
sider the OPA642 which offers considerably higher open
loop DC gain.
CROSSTALK
Crosstalk is the undesired result of the signal of one channel
mixing with and reproducing itself in the output of the other
channel. Crosstalk occurs in most multichannel integrated
circuits. In dual devices, the effect of crosstalk is measured by
driving one channel and observing the output of the undriven
channel over various frequencies. The magnitude of this effect
is referenced in terms of channel-to-channel crosstalk and
expressed in decibels. “Input referred” points to the fact that
there is a direct correlation between gain and crosstalk, there-
fore at increased gain, crosstalk also increases by a factor
equal to that of the gain. Figure 6 illustrates the measured
effect of crosstalk in the OPA2650U.
SPICE MODELS
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for Video and
RF amplifier circuits where parasitic capacitance and induc-
tance can have a major effect on circuit performance. SPICE
models are available on a disk from the Burr-Brown Appli-
cations Department.
DIFFERENTIAL GAIN AND PHASE
Differential Gain (dG) and Differential Phase (dP) are among
the more important specifications for video applications.
®
10
OPA2650
BB [ BURR-BROWN CORPORATION ]
