LT5514 datasheet(13/20 Pages) AD

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LT1993-10

Rev C

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Single-Ended to Differential Operation

The LT1993-10’s performance with single-ended inputs

is comparable to its performance with differential inputs.

This excellent single-ended performance is largely due to

the internal topology of the LT1993-10. Referring to the

block diagram, if the +INA and +INB pins are driven with

a single-ended signal (while –INA and –INB are tied to AC

ground), then the +OUT and –OUT pins are driven differ-

entially without any voltage swing needed from amplifier

C. Single-ended to differential conversion using more

conventional topologies suffers from performance lim-

itations due to the common mode amplifier.

Driving ADCs

The LT1993-10 has been specifically designed to inter-

face directly with high speed Analog to Digital Converters

(ADCs). In general, these ADCs have differential inputs,

with an input impedance of 1k or higher. In addition, there

is generally some form of lowpass or bandpass filter-

ing just prior to the ADC to limit input noise at the ADC,

thereby improving system signal to noise ratio. Both the

unfiltered and filtered outputs of the LT1993-10 can eas-

ily drive the high impedance inputs of these differential

ADCs. If the filtered outputs are used, then cutoff fre-

quency and the type of filter can be tailored for the specific

application if needed.

Wideband Applications

(Using the +OUT and –OUT Pins)

In applications where the full bandwidth of the LT1993-10

is desired, the unfiltered output pins (+OUT and –OUT)

should be used. They have a low output impedance; there-

fore, gain is unaffected by output load. Capacitance in

excess of 5pF placed directly on the unfiltered outputs

results in additional peaking and reduced performance.

When driving an ADC directly, a small series resistance

is recommended between the LT1993-10’s outputs and

the ADC inputs (Figure 4). This resistance helps eliminate

any resonances associated with bond wire inductances of

either the ADC inputs or the LT1993-10’s outputs. A value

between 10Ω and 25Ω gives excellent results.

Filtered Applications

(Using the +OUTFILTERED and –OUTFILTERED Pins)

Filtering at the output of the LT1993-10 is often desired

to provide either anti-aliasing or improved signal to noise

ratio. To simplify this filtering, the LT1993-10 includes an

additional pair of differential outputs (+OUTFILTERED and

–OUTFILTERED) which incorporate an internal lowpass fil-

ter network with a –3dB bandwidth of 175MHz (Figure 5).

These pins each have an output impedance of 25Ω. Internal

capacitances are 12pF to VEE 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

25Ω resistor with a 36pF capacitor shunting each filtered

output to AC ground, giving a –3dB bandwidth of 175MHz.

199310 F04

LT1993-10

–OUT

+OUT

10 TO 25

ADC

Figure 4. Adding Small Series R at LT1993-10 Output

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 +OUTFILTERED and the other between –OUT and –

OUTFILTERED (Figure 6). These resistors are in parallel

with the internal 25Ω resistor, lowering the overall resis-

tance and increasing filter bandwidth. To double the filter

bandwidth, for example, add two external 25Ω resistors to

lower the series resistance to 12.5Ω. The 36pF of capac-

itance remains unchanged, so filter bandwidth doubles.

Figure 5. LT1993-10 Internal Filter Topology –3dB BW ≈175MHz

VEE

199310 F05

12pF

LT1993-10

–OUT

+OUT

12pF

–OUTFILTERED

FILTERED OUTPUT

(350MHz)

+OUTFILTERED

APPLICATIONS INFORMATION

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