BESCHREIBUNG

The LT1993-2 is a low distortion, low noise Differential Amplifier/ADC driver for use in applications from DC to 800MHz. The LT1993-2 has been designed for ease of use, with minimal support circuitry required. Exceptionally low input-referred noise and low distortion products (with either single-ended or differential inputs) make the LT1993-2 an excellent solution for driving high speed 12-bit and 14-bit ADCs. In addition to the normal unfiltered outputs (+OUT and –OUT), the LT1993-2 has a built-in 175MHz differential low pass filter and an additional pair of filtered outputs (+OUTFILTERED, –OUTFILTERED) to reduce external filtering components when driving high speed ADCs. The output common mode voltage is easily set via the VOCM pin, eliminating either an output transformer or AC-coupling capacitors in many applications.

The LT1993-2 is designed to meet the demanding requirements of communications transceiver applications. It can be used as a differential ADC driver, a general-purpose differential gain block, or in any other application requiring differential drive. The LT1993-2 can be used in data acquisition systems required to function at frequencies down to DC.

The LT1993-2 operates on a 5V supply and consumes 100mA. It comes in a compact 16-lead 3 × 3 QFN package and operates over a –40°C to 85°C temperature range.

 

FEATURES

800MHz –3dB Bandwidth

Fixed Gain of 2V/V (6dB)

Low Distortion:

38dBm OIP3, –70dBc HD3 (70MHz, 2VP-P)

51dBm OIP3, –94dBc HD3 (10MHz, 2VP-P)

Low Noise: 12.3dB NF, en = 3.8nV/√Hz (70MHz)

Differential Inputs and Outputs

Additional Filtered Outputs

Adjustable Output Common Mode Voltage

DC- or AC-Coupled Operation

Minimal Support Circuitry Required

Small 0.75mm Tall 16-Lead 3 × 3 QFN Package

 

ANWENDUNGEN

Differential ADC Driver for:

Imaging

Communications

Differential Driver/Receiver

Single Ended to Differential Conversion

Differential to Single Ended Conversion

Level Shifting

IF Sampling Receivers

SAW Filter Interfacing/Buffering

 

ANWENDUNGSINFORMATIONEN

Circuit Description

The LT1993-2 is a low-noise, low-distortion differential amplifier/ADC driver with:

DC to 800MHz –3dB bandwidth

Fixed gain of 2V/V (6dB) independent of RLOAD

200 differential input impedance

Low output impedance

Built-in, user adjustable output filtering

Requires minimal support circuitry

The LT1993-2 uses a closed-loop topology which incorporates 3 internal amplifiers. Two of the amplifiers (A and B) are identical and drive the differential outputs. The third amplifier (C) is used to set the output common mode voltage. Gain and input impedance are set by the 200 resistors in the internal feedback network. Output impedance is low, determined by the inherent output impedance of amplifiers A and B, and further reduced by internal feedback.

The LT1993-2 also includes built-in single-pole output filtering. The user has the choice of using the unfiltered outputs, the filtered outputs (175MHz –3dB lowpass), or modifying the filtered outputs to alter frequency response by adding additional components. Many lowpass and bandpass filters are easily implemented with just one or two additional components.

The LT1993-2 has been designed to minimize the need for external support components such as transformers or AC-coupling capacitors. As an ADC driver, the LT1993-2 requires no external components except for power-supply bypass capacitors. This allows DC-coupled operation for applications that have frequency ranges including DC. At the outputs, the common mode voltage is set via the VOCM pin, allowing the LT1993-2 to drive ADCs directly. No output AC-coupling capacitors or transformers are needed. At the inputs, signals can be differential or single-ended with virtually no difference in performance. Furthermore, DC levels at the inputs can be set independently of the output common mode voltage. These input characteristics often eliminate the need for an input transformer and/or AC-coupling capacitors.

Single-Ended to Differential Operation

The LT1993-2’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 LT1993-2. 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 differentially without any voltage swing needed from amplifier C. Single-ended to differential conversion using more conventional topologies suffers from performance limitations due to the common mode amplifier.