EL4093CS datasheet(9/12 Pages) RENESAS

9 / 12 page

EL4093

FN7159 Rev 0.00

Page 9 of 12

January 1996

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

, dropping to 1M as the

autozero current increases to 3mA. Using RF = RG = 750,

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

, 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/750 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

 • CHOLD) = 1/(2 • (350)(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

). 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:

750

300

470

294

0.2

-1

680

300

GAIN

OPTIMUM RF

BW (MHz)

PEAKING (dB)

FIGURE 3.

IAZ

5.5mA

VDC

750



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







5.5mA

VDC 0.75V

–

750



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

VDC

750



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



GBWP

gm1

 CHOLD



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

gm2 RF





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