LTC1749 datasheet(13/20 Pages) LINER

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LTC1749

1749f

Input Drive Circuits

The LTC1749 requires differential drive for the analog

inputs. A balanced input drive will minimize even order

harmonics that are due to nonlinear behavior of the input

drive circuits and the S/H circuit.

The S/H circuit of the LTC1749 is a switched capacitor

circuit (Figure 2). The input drive circuitry will see a

sampling glitch at the start of the sampling period, when

ENC/ENC falls. Although designed to be linear as possible,

a small fraction of this glitch is nonlinear and can result in

additional observed distortion if the input drive circuitry is

too slow. For most practical circuits the glitch nonlinearity

is more than 100dB below the fundamental. The glitch will

decay during the sampling period with a time constant

determined by the input drive and S/H circuitry.

For fast settling and wide bandwidth, a low drive imped-

ance is required. The S/H bandwidth is partially deter-

mined by the source impedance. The full 500MHz

bandwidth is valid for source impedance (each input) less

than 30

Ω. Higher source impedance can be used but full

amplitude distortion will be better with a source imped-

ance less than 100

Ω.

Transformers

Transformers provide a simple method for converting a

single-ended signal to a differential signal; however, they

have poor performance characteristics at low and high input

frequencies. The lower –3dB corner of RF transformers can

range from tens of kHz to tens of MHz. Operation near this

corner results in poor 2nd order harmonic performance

due to nonlinear transformer core behavior. The upper

–3dB corner can vary from tens of MHz to several GHz.

Operation near the upper corner can result in poor 2nd order

performance due to poor balance on the secondary.

Transformers should be selected to have –3dB corners at

least one octave away from the desired operating fre-

quency. Transformers with larger cores usually have

better performance at lower frequency and perform better

when driving heavy loads.

Figure 3a shows the LTC1749 being driven by an RF

transformer with a center tapped secondary. The second-

ary center tap is DC-biased with VCM, setting the ADC input

signal at its optimum DC level of 2V. In this example a 1:1

transformer is used; however, other transformer imped-

ance ratios may be substituted.

Figure 3b shows the use of a transformer without a center

tapped secondary. In this example the secondary is biased

with the addition of two resistors placed in series across

the secondary winding. The center tap of the secondary

resistors is connected to the ADC VCM output to set the DC

bias. This circuit is better suited for high input frequency

applications since center tapped transformers generally

have less bandwidth and poor balance at high frequencies

than noncenter tapped transformers.

APPLICATIO S I FOR ATIO

Figure 3a. Single-Ended to Differential

Conversion Using a Transformer

1:1

Ω

0.1

µF

ANALOG

INPUT

100

Ω

100

Ω

12pF

1749 F03

4.7

µF

Ω

LTC1749

VCM

AIN

–

1:4

Ω

0.1

µF

ANALOG

INPUT

100

Ω

200

Ω

200

Ω

8.4pF

1749 F03b

Ω

8.4pF

4.7

µF

LTC1749

VCM

AIN

–

Figure 3b. Using a Transformer

Without a Center Tapped Secondary

Active Drive Circuits

Active circuits, open loop or closed loop, can be used to

drive the ADC inputs. Closed-loop circuits such as op amps

have excellent DC and low frequency accuracy but have

poor high frequency performance. Figure 4 shows the dual

®1818 op amp used for single-ended to differential

signal conversion. Note that the two op amps do not have

the same noise gain, which can result in poor balance at

higher frequencies. The op amp configured in a gain of +1

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