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LTC2201

2201f

Figure 12. Functional Equivalent Block Diagram of

Internal Dither Circuit

S/H

AMP

DIGITAL

SUMMATION

OUTPUT

DRIVERS

MULTIBIT DEEP

PSEUDO-RANDOM

NUMBER

GENERATOR

16-BIT

PIPELINED

ADC CORE

PRECISION

DAC

CLOCK/DUTY

CYCLE

CONTROL

CLKOUT+

CLKOUT–

D15

CLK

DITHER ENABLE

HIGH = DITHER ON

LOW = DITHER OFF

DITH

ANALOG

AIN

–

INPUT

2201 F12

LTC2201

Internal Dither

The LTC2201 is a 16-bit ADC with a very linear transfer

function; however, at low input levels even slight imperfec-

tions in the transfer function will result in unwanted tones.

Small errors in the transfer function are usually a result

of ADC element mismatches. An optional internal dither

mode can be enabled to randomize the input’s location

on the ADC transfer curve, resulting in improved SFDR

for low signal levels.

As shown in Figure 12, the output of the sample-and-hold

ampliﬁer is summed with the output of a dither DAC. The

dither DAC is driven by a long sequence pseudo-random

number generator; the random number fed to the dither

DAC is also subtracted from the ADC result. If the dither

DAC is precisely calibrated to the ADC, very little of the

dither signal will be seen at the output. The dither signal

that does leak through will appear as white noise. The dither

DAC is calibrated to result in less than 0.5dB elevation in

the noise ﬂoor of the ADC, as compared to the noise ﬂoor

with dither off.

Grounding and Bypassing

The LTC2201 require a printed circuit board with a

clean unbroken ground plane; a multilayer board with an

internal ground plane is recommended. The pinout of the

LTC2201 has been optimized for a ﬂowthrough layout so

that the interaction between inputs and digital outputs is

minimized. Layout for the printed circuit board should

ensure that digital and analog signal lines are separated

as much as possible. In particular, care should be taken

not to run any digital track alongside an analog signal

track or underneath the ADC.

High quality ceramic bypass capacitors should be used

at the VDD, VCM, and OVDD pins. Bypass capacitors must

be located as close to the pins as possible. The traces

connecting the pins and bypass capacitors must be kept

short and should be made as wide as possible.

The LTC2201 differential inputs should run parallel and

close to each other. The input traces should be as short

as possible to minimize capacitance and to minimize

noise pickup.

Heat Transfer

Most of the heat generated by the LTC2201 is transferred

from the die through the bottom-side exposed pad. For

good electrical and thermal performance, the exposed

pad must be soldered to a large grounded pad on the PC

board. It is critical that the exposed pad and all ground

pins are connected to a ground plane of sufﬁcient area

with as many vias as possible.

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