DC-Restore Circuit

Comparator

20

Ω

20

Ω

150

Ω

+1

R

2

100k

Ω

V

IN

6

5

R

2

100

Ω

R

1

40.2

Ω

CCII

B

3

2

E

C

8

V

OUT

V

REF

C

1

100pF

D

1

D

2

OPA656

The OTA amplifier works as a current conveyor (CCII) in this circuit, with a current gain of 1.

R

1 and C1 set the DC restoration time constant.

D

1 adds a propogation delay to the DC restoration.

R

2 and C1 set the decay time constant.

D

1, D2 = 1N4148

R

Q = 1kΩ

OPA860

www.ti.com ....................................................................................................................................................... SBOS331C – JUNE 2005 – REVISED AUGUST 2008

The OPA860 can be used advantageously with an

An interesting and also cost-effective circuit solution

operational

amplifier,

here

the

OPA820,

as

a

using the OPA860 as a low-jitter comparator is shown

DC-restore circuit. Figure 56 illustrates this design.

in Figure 57. At the same time, this circuit uses a

Depending

on

the

collector

current

of

the

positive

and

negative

feedback.

The

input

is

transconductance amplifier (OTA) of the OPA860, a

connected to the inverting E-input. The output signal

switching function is realized with the diodes D1 and

is applied in a direct feedback over the two

D2.

antiparallel, connected gallium-arsenide diodes back

to the emitter. A second feedback path over the RC

When the C-output is sourcing current, the capacitor

combination

to

the

base,

which

is

a

positive

C1 is being charged. When the C-output is sinking

feedback, accelerates the output voltage change

current, D1 is turned off and D2 is turned on, letting

when the input voltage crosses the threshold voltage.

the voltage across C1 be discharged through R2.

The output voltage is limited to the threshold voltage

of the back-to-back diodes.

The condition to charge C1 is set by the voltage

difference between VREF and VOUT. For the OTA

C-output to source current, VREF has to be greater

than VOUT. The rate of charge of C1 is set by both R1

and C1. The discharge rate is given by R2 and C1.

Figure 56. DC Restorer Circuit

Copyright © 2005–2008, Texas Instruments Incorporated

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