EL4093CS datasheet(9/12 Pages) INTERSIL

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Autozero Interface

The autozero interface refers to the connection between the

S/H output and the CFA inverting input. This interface has

been greatly simplified compared to that of the EL2090, in

that the S/H output is a high impedance current source. The

S/H output can be connected directly to the inverting input,

and its high impedance greatly reduces the interaction

between the sample & hold and the gain setting resistors.

Another virtue of this interface is better gain linearity as the

autozero current changes. For example, at an autozero

current of 0mA the output impedance is about 5M

Ω,

dropping to 1M

Ω as the autozero current increases to 3mA.

Using RF = RG = 750Ω, the closed loop gain changes only

by 0.025% in this interval.

Autozero Range

The autozero range is defined as the difference between the

input DC level and the reference voltage to restore to. The

size of this range is a function of the gain setting resistors

used and the S/H output current swing. For a gain of +2 the

optimum feedback resistor is 750

Ω, and the available S/H

output current is ±5.5mA minimum. To determine the

autozero range for this case, we refer to Figure 3 below.

Suppose that the input DC level is +VDC, and that the

reference voltage is 0V. We know that in feedback, the

following two conditions will exist on the CFA: first, its output

will be equal to 0V (due to autozero), and second, its VIN-

voltage is equal to the VIN+ voltage (i.e. VIN- = +VDC). So

we have a potential difference of +VDC across both RF and

RG, resulting in a current IRF = IRG = VDC/750Ω that must

flow into each of them. This current IAZ = (IRF + IRG) must

come from the S/H output. Since the maximum that IAZ can

be is 5.5mA, we can solve for VDC using the following:

and see that VDC = ±2V. This range can easily

accommodate most video signals.

As another example, consider the case where we are

restoring to a reference voltage of +0.75V. Using the same

reasoning as above, a current IRF = (VDC - 0.75V)/RF must

flow through RF, and a current IRG = VDC/RG must go into

RG. Again, our boundary condition is that IRF + IRG

≤ ±5.5mA, and we can solve for the allowable VDC values

using the following:

Hence VDC must be between +2.4V to -1.7V. This example

illustrates that when the reference changes, the autozero

range also changes. In general, the user should determine

the autozero range for his/her application, and ensure that

the input signal is within this range during the autozero

period.

Autozero Loop Bandwidth

The gain-bandwidth product (GBWP) of the autozero loop is

determined by the size of the hold capacitor, the value of RF,

and the transconductances (gm’s) of the S/H amplifier. To

begin, the S/H amplifier is modeled as in Figure 4. First, the

input stage transconductance is represented by gm1, with

the compensation capacitor given by CHOLD. This stage’s

GBWP is thus gm1/(2

π • C

HOLD) = 1/(2π • (350Ω)(2.2nF)) =

207kHz. Next, since the S/H has a current output, its output

stage can be modeled as a transconductance gm2, in this

case having a value of 1/(500

Ω). The current from gm2 then

flows through the I to V converter made up of the CFA and

RF to produce a voltage gain. Thus the GBWP of the overall

loop is given by:

GAIN

OPTIMUM RF

BW (MHz)

PEAKING (dB)

910

314

0.2

750

300

470

294

0.2

-1

680

300

FIGURE 3.

5.5mA

750

Ω

---------------





5.5mA

0.75V

–

750

Ω

----------------------------------

750

Ω

---------------

GBWP

gm1

π C

HOLD

--------------------------------- gm2

()

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