ICL7650CPA 查看數據表（PDF） - Linear Technology

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ICL7650CPA

Zero-Drift Operational Amplifier

Linear Technology 

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THEORY OF OPERATIO

LTC1052/LTC7652

power supply are also nulled. For nulling to take place, the

offset voltage, common mode voltage and power supply

must not change at a frequency which is high compared to

the frequency response of the nulling loop.

AC OPERATION AND ALIASING ERRORS

So far, the DC performance of the LTC1052 has been

explained. As the input signal frequency increases, the

problem of aliasing must be addressed. Aliasing is the

spurious formation of low and high frequency signals

caused by the mixing of the input signal with the sampling

frequency, fS. The frequency of the error signals, fE, is:

fE = fS ±fI

where fI = input signal frequency.

Normally it is the difference frequency (fS – fI ) which is of

concern because the high frequency (fS + fI) can be easily

filtered. As the input frequency approaches the sampling

frequency, the difference frequency approaches zero and

will cause DC errors—the exact problem that the zero-drift

amplifier is meant to eliminate.

The solution is simple; filter the input so the sampling loop

never sees any frequency near the sampling frequency.

At a frequency well below the sampling frequency, the

LTC1052 forces I1 to equal I2 (see Figure 1b). This makes

δ l zero, thus the gain of the sampling loop zero at this and

higher frequencies (i.e., a low pass filter). The corner

frequency of this low pass filter is set by the output stage

pole (1/RL4 gm5 RL5 C2).

For frequencies above this pole, I2 is:

I2 = VIN gm6 •

1

SC2

• SC1

and

I1 – I2 = VIN gm1 – VIN gm6 •

C1

C2

The LTC1052 is very carefully designed so that gm1 = gm6

and C1 = C2. Substituting these values in the above equa-

tion shows I1 – I2 = 0.

The gm6 input stage, with Cl and C2, not only filters the

input to the sampling loop, but also acts as a high

frequency path to give the LTC1052 good high frequency

response. The unity-gain cross frequencies for both the

DC path and high frequency path are identical

[f3dB =

1

2π

(gm1/C1) =

1

2π

(gm6/C2)]

thereby making the frequency response smooth and con-

tinuous while eliminating sampling noise in the output as

the loop transitions from the high gain DC loop to the high

frequency loop.

The typical curves show just how well the amplifier works.

The output spectrum shows that the difference frequency

(fI–fS = 100Hz) is down by 80dB and the frequency

response curve shows no abnormalities or perturbations.

Also note the well-behaved small and large-signal step

responses and the absence of the sampling frequency in

the output spectrum. If the dynamics of the amplifier

(i.e., slew rate and overshoot), depend on the sampling

clock, the sampling frequency will appear in the output

spectrum.

C1

S3

VREF

C2

+ IN

S1

– IN

+

gm1

–

+–

–

RL1

gm2

gm3

S2

RL2

VNULL

CEXT B

CEXT A

gm4

+–

+

gm5

RL4

VOUT

RL5

–

gm6

+

V–

Figure 1a. LTC1052 Block Diagram

Auto Zero Cycle

LTC1052/7652 • TPC13

1052fa

7

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