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LTC2206

16-Bit, 105Msps/80Msps ADCs

Linear Technology 

LTC2207/LTC2206

APPLICATIONS INFORMATION

Direct Coupled Circuits

Figure 5 demonstrates the use of a differential amplifier to

convert a single ended input signal into a differential input

signal. The advantage of this method is that it provides

low frequency input response; however, the limited gain

bandwidth of any op amp or closed-loop amplifier will de-

grade the ADC SFDR at high input frequencies. Additionally,

wideband op amps or differential amplifiers tend to have

high noise. As a result, the SNR will be degraded unless

the noise bandwidth is limited prior to the ADC input.

Reference Operation

Figure 6 shows the LTC2207/LTC2206 reference circuitry

consisting of a 2.5V bandgap reference, a programmable

gain amplifier and control circuit. The LTC2207/LTC2206

have three modes of reference operation: Internal Refer-

ence, 1.25V external reference or 2.5V external reference.

To use the internal reference, tie the SENSE pin to VDD. To

use an external reference, simply apply either a 1.25V or

2.5V reference voltage to the SENSE input pin. Both 1.25V

and 2.5V applied to SENSE will result in a full scale range

of 2.25VP-P (PGA = 0). A 1.25V output VCM is provided

for a common mode bias for input drive circuitry. An

external bypass capacitor is required for the VCM output.

This provides a high frequency low impedance path to

ground for internal and external circuitry. This is also the

compensation capacitor for the reference; it will not be

stable without this capacitor. The minimum value required

for stability is 2.2µF.

The internal programmable gain amplifier provides the

internal reference voltage for the ADC. This amplifier has

very stringent settling requirements and is not accessible

for external use.

The SENSE pin can be driven ±5% around the nominal 2.5V

or 1.25V external reference inputs. This adjustment range

can be used to trim the ADC gain error or other system

gain errors. When selecting the internal reference, the

SENSE pin should be tied to VDD as close to the converter

as possible. If the sense pin is driven externally it should

be bypassed to ground as close to the device as possible

with 1µF (or larger) ceramic capacitor.

PGA Pin

The PGA pin selects between two gain settings for the

ADC front-end. PGA = 0 selects an input range of 2.25VP-

P; PGA = 1 selects an input range of 1.5VP-P. The 2.25V

input range has the best SNR; however, the distortion will

be higher for input frequencies above 100MHz. For ap-

plications with high input frequencies, the low input range

will have improved distortion; however, the SNR will be

2.4dB worse. See the Typical Performance Characteristics

section of this datasheet.

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER 25Ω

++

CM

––

25Ω

AMPLIFIER = LTC6600-20,

LTC1993, ETC.

VCM

2.2µF

AIN+

12pF

LTC2207/

LTC2206

AIN–

12pF

22076 F05

Figure 5. DC Coupled Input with Differential Amplifier

LTC2207/

LTC2206

TIE TO VDD TO USE

INTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 2.5V

REFERENCE

OR INPUT FOR

EXTERNAL 1.25V

REFERENCE

SENSE

RANGE

SELECT

AND GAIN

CONTROL

VCM

2.2µF

BUFFER

1.25V

INTERNAL

ADC

REFERENCE

PGA

2.5V

BANDGAP

REFERENCE

22076 F06

Figure 6. Reference Circuit

22076fa

20

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