Broadband Ultra Low Distortion 7-Bit Digitally Controlled VGA

The LT5554 is a 7-bit digitally controlled programmable gain (PG) amplifier with 16dB gain control range. It consists of a 50Ω input variable attenuator, followed by a high linearity variable transconductance amplifier. The coarse 4dB input attenuator step is implemented via 2-bits of digital control (PG5, PG6). The fine transconductance amplifier 0.125dB step within 3.875dB gain control range is set via 5-bits digital control (PG0 to PG4). The LT5554 gain control inputs (PGx) and the STROBE input can be directly coupled to TTL or ECL drivers. The seven parallel gain control inputs time skew can be eliminated by using the STROBE input positive transition.

The internal output resistor RO = 400Ω limits the maximum overall gain to 36dB for open outputs. The internal circuitry of open output collectors enables the LT5554 to be unconditionally stable over any loading conditions (including external SAW filters) and provides –80dB reverse isolation at 300MHz.

The LT5554 is internally protected during overdrive and has an on-chip power supply regulator.

With 0.125dB step resolution and 5ns settling time, the LT5554 is suitable in applications where continuous gain control is required.

Applications

* Differential ADC Driver

* IF Sampling Receivers

* VGA IF Power Amplifier

* 50Ω Driver

* Instrumentation

* 1GHz Bandwidth at all Gains

* 48dBm OIP3 at 200MHz, 2VP-P into 50Ω, ROUT = 100Ω

* –88dBc IMD3 at 200MHz, 2VP-P into 50Ω, ROUT = 100Ω

* 1.4nV/√Hz Input-Referred-Noise (RTI)

* 20dBm Output P1dB at 70MHz, ROUT = 130Ω

* 2dB to 18dB Gain Range (ROUT = 50Ω)

* 0.125dB Gain Step Size

* 30ps Group Delay Variation

* 5ns Fast Gain Settling Time

* 5ns Fast Overdrive Recovery

* –80dB Reverse Isolation