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LT1993-2

Rev B

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Output Common Mode Adjustment

The LT1993-2’s output common mode voltage is set by

the VOCM pin. It is a high-impedance input, capable of

setting the output common mode voltage anywhere in a

range from 1.1V to 3.6V. Bandwidth of the VOCM pin is

typically 300MHz, so for applications where the VOCM pin

is tied to a DC bias voltage, a 0.1µF capacitor at this pin

is recommended. For best distortion performance, the

voltage at the VOCM pin should be between 1.8V and 2.6V.

When interfacing with most ADCs, there is generally a

VOCM output pin that is at about half of the supply voltage

of the ADC. For 5V ADCs such as the LTC17XX family,

this VOCM output pin should be connected directly (with

the addition of a 0.1µF capacitor) to the input VOCM pin of

the LT1993-2. For 3V ADCs such as the LTC22XX fami-

lies, the LT1993-2 will function properly using the 1.65V

from the ADC’s VCM reference pin, but improved Spurious

Free Dynamic Range (SFDR) and distortion performance

can be achieved by level-shifting the LTC22XX’s VCM ref-

erence voltage up to at least 1.8V. This can be accom-

plished as shown in Figure 10 by using a resistor divider

between the LTC22XX’s VCM output pin and VCC and then

bypassing the LT1993-2’s VOCM pin with a 0.1µF capaci-

tor. For a common mode voltage above 1.9V, AC coupling

capacitors are recommended between the LT1993-2 and

LTC22XX ADCs because of the input voltage range con-

straints of the ADC.

Large Output Voltage Swings

The LT1993-2 has been designed to provide the 3.2VP-P

output swing needed by the LTC1748 family of 14-bit

low-noise ADCs. This additional output swing improves

system SNR by up to 4dB. Typical performance curves

and AC specifications have been included for these

applications.

Input Bias Voltage and Bias Current

The input pins of the LT1993-2 are internally biased to

the voltage applied to the VOCM pin. No external biasing

resistors are needed, even for AC-coupled operation. The

input bias current is determined by the voltage difference

between the input common mode voltage and the VOCM

pin (which sets the output common mode voltage). At

both the positive and negative inputs, any voltage differ-

ence is imposed across 200 , generating an input bias

current. For example, if the inputs are tied to 2.5V with

the VOCM pin at 2.2V, then a total input bias current of

1.5mA will flow into the LT1993-2’s +INA and +INB pins.

Furthermore, an additional input bias current totaling

1.5mA will flow into the –INA and –INB inputs.

VOCM and Output Voltages for Proper Operation

The electrical tables suggest 1.3V VOCM minimum to

ensure proper operation over temperature. The electrical

tables also guarantee VSWINGMIN to be 0.5V over tem-

perature. It would appear that operation is okay so long

as the low output voltage on either pin is 0.5V or higher,

and VOCM is 1.3V or higher. The values in the table are

correct. However, the VSWINGMIN value is tested with the

input difference overdriven. Overdriven input is a static

condition during which the overdrive forces transistors

well into their saturated state.

Amplifier Inputs

Dynamically, the situation is more complicated. Referring

to the Block Diagram, an NPN differential pair with tail

current forms the input stage of both amplifiers A and

B. Appropriate biasing of these inputs requires 1.3V, or

in other words the same value as the minimum VOCM.

19932 F10

IF IN

LT1993-2

–INA

–INB

VOCM

+INB

+INA

80.6

Ω

LTC22xx

0.1

µF

0.1

µF

+OUTFILTERED

–OUTFILTERED

AIN+

AIN–

4.02k

11k

1.9V

1.5V

VCM

Figure10. Level Shifting 3V ADC VCM Voltage for

Improved SFDR

APPLICATIONS INFORMATION

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