10

®

OPA2650

FREQUENCY RESPONSE COMPENSATION

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 = (1/2

πR

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.

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

reacts with the parasitic capacitance at the summing node.

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

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.

DIFFERENTIAL GAIN AND PHASE

Differential Gain (dG) and Differential Phase (dP) are among

the more important specifications for video applications.

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.

DISTORTION

The OPA2650’s harmonic distortion characteristics into a

100

Ω 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.

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.

–60

–70

–80

–90

10

20

50

100

200

500

1k

Load Resistance (

Ω)

(G = +1, f

2f

O

3f

O

FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.