LTC6404 datasheet(23/28 Pages) LINER

23 / 28 page

LTC6404

6404f

In single supply applications, where the LTC6404 is used

to interface to an ADC, the optimal common mode input

range to the ADC is often determined by the ADC’s refer-

ence. If the ADC makes a reference available for setting

the input common mode voltage, it can be directly tied

to the VOCM pin, but must be capable of driving the input

impedance presented by the VOCM as listed in the Electri-

cal Characteristics Table. This impedance can be assumed

to be connected to a mid-supply potential. If an external

reference drives the VOCM pin, it should still be bypassed

with a high quality 0.01μF or larger capacitor to a low

impedance ground plane to ﬁlter any thermal noise and

to prevent common mode signals on this pin from being

inadvertently converted to differential signals.

Output Filter Considerations and Use

Filtering at the output of the LTC6404 is often desired to

provide either anti-aliasing or improved signal to noise

ratio. To simplify this ﬁltering, the LTC6404 includes an

additional pair of differential outputs (OUTF+ and OUTF–)

which incorporate an internal lowpass ﬁlter network with

a –3dB bandwidth of 88.5MHz (Figure 6).

These pins each have a DC output impedance of 50

Ω. In-

ternal capacitances are 12pF to V– on each ﬁltered output,

plus an additional 12pF capacitor connected differentially

between the two ﬁltered outputs. This resistor/capacitor

combination creates ﬁltered outputs that look like a series

Ω resistor with a 36pF capacitor shunting each ﬁltered

output to AC ground, providing a –3dB bandwidth of

APPLICATIONS INFORMATION

88.5MHz, and a noise bandwidth of 139MHz. The ﬁlter

cutoff frequency is easily modiﬁed with just a few external

components. To increase the cutoff frequency, simply add 2

equal value resistors, one between OUT+ and OUTF+ and the

other between OUT– and OUTF– (Figure 7). These resistors,

in parallel with the internal 50

Ω resistor, lower the overall

resistance and therefore increase ﬁlter bandwidth. For

example, to double the ﬁlter bandwidth, add two external

Ω resistors to lower the series ﬁlter resistance to 25Ω.

The 36pF of capacitance remains unchanged, so ﬁlter

bandwidth doubles. Keep in mind, the series resistance

also serves to decouple the outputs from load capacitance.

The unﬁltered outputs of the LTC6404 are designed to

drive 10pF to ground or 5pF differentially, so care should

be taken to not lower the effective impedance between

OUT+ and OUTF+ or OUT– and OUTF– below 25

Ω.

To decrease ﬁlter bandwidth, add two external capacitors,

one from OUTF+ to ground, and the other from OUTF– to

ground. A single differential capacitor connected between

OUTF+ and OUTF– can also be used, but since it is being

driven differentially it will appear at each ﬁltered output

as a single-ended capacitance of twice the value. To halve

the ﬁlter bandwidth, for example, two 36pF capacitors

could be added (one from each ﬁltered output to ground).

Alternatively, one 18pF capacitor could be added between

the ﬁltered outputs, again halving the ﬁlter bandwidth.

Combinations of capacitors could be used as well; a three

Figure 7. LTC6404 Filter Topology Modiﬁed for 2x Filter

Bandwidth (2 External Resistors)

Figure 6. LTC6404 Internal Filter Topology

–

OUT+

OUTF+

OUT–

OUTF–

V–

6404 F06

LTC6404

FILTERED OUTPUT

(88.5MHz)

50Ω

12pF

50Ω

–

OUT+

OUTF+

OUT–

OUTF–

V–

6404 F07

LTC6404

FILTERED OUTPUT

(176MHz)

50Ω

49.9Ω

12pF

50Ω

49.9Ω

LTC6404 Datasheet(PDF) 23 Page - Linear Technology

LTC6404 Datasheet(HTML) 23 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL