LTC1062(RevD) 查看數據表（PDF） - Linear Technology

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LTC1062
(Rev.:RevD)

5th Order Lowpass Filter

Linear Technology 

LTC1062

APPLICATIO S I FOR ATIO

Filter Input Voltage Range

Every node of the LTC1062 typically swings within 1V of

either voltage supply, positive or negative. With the appro-

priate external (RC) values, the amplitude response of all

the internal or external nodes does not exceed a gain of

0dB with the exception of Pin 1. The amplitude response

of the feedback node (Pin 1) is shown in Figure 2. For an

input frequency around 0.8 • fC, the gain is 1.7V/V and, with

±5V supplies, the peak-to-peak input voltage should not

exceed 4.7V. If the input voltage goes beyond this value,

clipping and distortion of the output waveform occur, but

the filter will not get damaged nor will it oscillate. Also, the

absolute maximum input voltage should not exceed the

power supplies.

6

4

2

0

–2

–4

–6

–8

–10

–12

–14

0.1

VS = ±5V

1

2πRC

=

fC

1.62

1

fIN/fC

10

1062 F02

Figure 2. Amplitude Response of Pin 1

Internal Buffer

The internal buffer out (Pin 8) and Pin 1 are part of the

signal AC path. Excessive capacitive loading will cause

gain errors in the passband, especially around the cutoff

frequency. The internal buffer gain at DC is typically

0.006dB. The internal buffer output can be used as a filter

output, however, it has a few millivolts of DC offset. The

temperature coefficient of the internal buffer is typically

1µV/°C.

Filter Attenuation

The LTC1062 rolloff is typically 30dB/octave. When the

clock and the cutoff frequencies increase, the filter’s

maximum attenuation decreases. This is shown in the

6

Typical Performance Characteristics. The decrease of the

maximum attenuation is due to the rolloff at higher

frequencies of the loop gains of the various internal

feedback paths and not to the increase of the noise floor.

For instance, for a 100kHz clock and 1kHz cutoff fre-

quency, the maximum attenuation is about 64dB. A 4kHz,

1VRMS input signal will be predictably attenuated by 60dB

at the output. A 6kHz, 1VRMS input signal will be attenu-

ated by 64dB and not by 77dB as an ideal 5th order

maximum flat filter would have dictated. The LTC1062

output at 6kHz will be about 630µVRMS. The measured

RMS noise from DC to 17kHz was 100µVRMS which is

16dB below the filter output.

COSC, Pin 5

The COSC, Pin 5, can be used with an external capacitor,

COSC, connected from Pin 5 to ground. If COSC is polarized

it should be connected from Pin 5 to the negative supply,

Pin 3. COSC lowers the internal oscillator frequency. If

Pin 5 is floating, an internal 33pF capacitor plus the

external interpin capacitance set the oscillator frequency

around 140kHz with ±5V supply. An external COSC will

bring the oscillator frequency down by the ratio (33pF)/

(33pF + COSC). The Typical Performance Characteristics

curves provide the necessary information to get the inter-

nal oscillator frequency for various power supply ranges.

Pin 5 can also be driven with an external CMOS clock to

override the internal oscillator. Although standard 7400

series CMOS gates do not guarantee CMOS levels with the

current source and sink requirements of Pin 5, they will, in

reality, drive the COSC pin. CMOS gates conforming to

standard B series output drive have the appropriate volt-

age levels and more than enough output current to

simultaneously drive several LTC1062 COSC pins. The

typical trip levels of the internal Schmitt trigger which

input is Pin 5, are given in Table 1.

Table 1

VSUPPLY

±2.5V

±5V

±6V

±7V

VTH+

0.9V

1.3V

1.7V

1.75V

VTH–

–1V

–2.1V

–2.5V

–2.9V

1062fd

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