DESCRIPCIÓN GENERAL

The AD8007 (single) and AD8008 (dual) are high performance current feedback amplifiers with ultralow distortion and noise. Unlike other high performance amplifiers, the low price and low quiescent current allow these amplifiers to be used in a wide range of applications. Analog Devices, Inc., proprietary second-generation eXtra-Fast Complementary Bipolar (XFCB) process enables such high performance amplifiers with low power consumption.

The AD8007/AD8008 have 650 MHz bandwidth, 2.7 nV/√Hz voltage noise, −83 dB SFDR at 20 MHz (AD8007), and −77 dBc SFDR at 20 MHz (AD8008).

With the wide supply voltage range (5 V to 12 V) and wide bandwidth, the AD8007/AD8008 are designed to work in a variety of applications. The AD8007/AD8008 amplifiers have a low power supply current of 9 mA/amplifier.

The AD8007 is available in a tiny SC70 package as well as a standard 8-lead SOIC. The AD8007 is rated to work over the industrial temperature range of −40°C to +85°C.

 

CARACTERÍSTICAS

Extremely low distortion

Second harmonic

−88 dBc @ 5 MHz

−83 dBc @ 20 MHz (AD8007)

−77 dBc @ 20 MHz (AD8008)

Third harmonic

−101 dBc @ 5 MHz

−92 dBc @ 20 MHz (AD8007)

−98 dBc @ 20 MHz (AD8008)

High speed

650 MHz, −3 dB bandwidth (G = +1)

1000 V/μs slew rate

Low noise

2.7 nV/√Hz input voltage noise

22.5 pA/√Hz input inverting current noise

Low power: 9 mA/amplifier typical supply current

Wide supply voltage range: 5 V to 12 V

0.5 mV typical input offset voltage

Small packaging: 8-lead SOIC, 8-lead MSOP, and 5-lead SC70

 

APLICACIONES

Instrumentación

IF and baseband amplifiers

Filters

A/D drivers

DAC buffers

 

TEORÍA DE FUNCIONAMIENTO

The AD8007 (single) and AD8008 (dual) are current feedback amplifiers optimized for low distortion performance. It closely resembles a classic current feedback amplifier comprised of a complementary emitter-follower input stage, a pair of signal mirrors, and a diamond output stage. However, in the case of the AD8007/AD8008, several modifications were made to improve the distortion performance over that of a classic current feedback topology.

The signal mirrors were replaced with low distortion, high precision mirrors. Their primary function from a distortion standpoint is to reduce the effect of highly nonlinear distortion caused by capacitances, CJ1 and CJ2. These capacitors represent the collector-to-base capacitances of the output devices of the mirrors.

A voltage imbalance arises across the output stage, as measured from the high impedance node, high-Z, to the output node, OUT. This imbalance is a result of delivering high output currents and is the primary cause of output distortion. Circuitry is included to sense this output voltage imbalance and generate a compensating current, IDO. When injected into the circuit, IDO reduces the distortion that could be generated at the output stage. Similarly, the nonlinear voltage imbalance across the input stage (measured from the noninverting to the inverting input) is sensed, and a current, IDI, is injected to compensate for input-generated distortion.

The design and layout are strictly top-to-bottom symmetric to minimize the presence of even-order harmonics.

USING THE AD8007/AD8008

Supply Decoupling for Low Distortion

Decoupling for low distortion performance requires careful consideration. The commonly adopted practice of returning the high frequency supply decoupling capacitors to physically separate (and possibly distant) grounds can lead to degraded even-order harmonic performance. For a sinusoidal input, each decoupling capacitor returns to its ground a quasi-rectified current carrying high even-order harmonics.

The two high frequency decoupling capacitors are first tied together at a common node and are then returned to the ground plane through a single connection. By first adding the two currents flowing through each high frequency decoupling capacitor, this ensures that the current returned into the ground plane is only at the fundamental frequency.

Whenever physical layout considerations prevent the decoupling, the user can connect one of the high frequency decoupling capacitors directly across the supplies and connect the other high frequency decoupling capacitor to ground.