OBSOLETE

CLC404

SNOS851F – MAY 1999 – REVISED APRIL 2013

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The CLC404 reaches slew rate limits only for low non-inverting gains. In other words, slew rate limiting is

constrained by common mode voltage swings at the input. (This is different from traditional slew rate constraints.)

The large-signal frequency response plot at a gain of +2 shows a break in the response, which shows that slew

rate limit has been reached. Note also that the frequency response plots at gain of +21 show that the large signal

and small signal responses are nearly identical.

Differential Gain and Phase

Differential gain and phase are measurements useful primarily in composite video channels. Differential gain and

phase are measured by monitoring the gain and phase of a high frequency carrier (3.58MHz for NTSC composite

video) as the output of the amplifier is swept over a range of DC voltages. Any changes in gain and phase at the

carrier frequency are the desired measurement, differential gain and phase.

video level. Test conditions used are the following.

DC sweep range: 0 to 100 IRE units (black to white)

Carrier: 3.58MHz at 40 IRE units peak to peak

The amplifier is specified for a gain of +2, and 150

Ω load (for a backmatched 75Ω system.) IRE amplitudes are

referred to 75

Source Impedance

For best results, source impedance in the non-inverting circuit configuration (see Figure 18) should be kept below

3k

Ω Above 3kΩ it is possible for oscillation to occur, depending on other circuit parasitics. Depending on the

signal source, a resistor with a value of less than 3k

Ω may be used to terminate the non-inverting input to

ground.

Feedback Resistor

In current-feedback op amps, the value of the feedback resistor plays a major role in determining amplifier

dynamics. It is important to select the correct value resistor. The CLC404 provides optimum performance with a

500

Ω feedback resistor. Furthermore, the specifications shown on the previous pages are valid only when a

500

Ω feedback resistor is used. Selection of an incorrect value can lead to severe rolloff in frequency-response

(if the resistor value is too large) or peaking or oscillation (if the value is too low).

Printed Circuit Layout

As with any high frequency device, a good PCB layout will enhance performance. Ground plane construction and

good power supply bypassing close to the package are critical to achieving full performance. In the non-inverting

configuration, the amplifier is sensitive to stray capacitance to ground at the inverting input. Hence, the inverting

node connections should be small with minimal coupling to the ground plane. Shunt capacitance across the

feedback resistor should not be used to compensate for this effect.

Parasitic or load capacitance directly on the output will introduce additional phase shift in the loop degrading the

loop phase margin and leading to frequency response peaking. A small series resistor before the capacitance

effectively decouples this effect. The graphs on the preceding page illustrate the required resistor value and

resulting performance vs. capacitance.

Precision buffed resistors (PRP8351 series from Precision Resistive Products) with low parasitic reactances were

used to develop the data sheet specifications. Precision carbon composition resistors will also yield excellent

results. Standard spirally-trimmed RN55D metal film resistors will work with a slight decrease in bandwidth due to

their reactive nature at high frequencies.

Evaluation PC boards (part numbers CLC730013 for through-hole and CLC 730027 for SOIC) for the CLC404

are available.

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