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Product of the Month: May '99
Data Sheet | Analog Devices' Active Transformer

Analog Devices AD8138 ADC Driver
Differential ADC Driver Maximizes Distortion Performance In ADCs and Current-output DACs

The manufacturer says . . . Chipcenter's Paul McGoldrick says . . .

Analog Devices, Inc. announced the general availability of a new low distortion differential ADC driver that maximizes the distortion performance in high-performance differential ADCs and current-output DACs. Analog Devices is now shipping the AD8138 to leading manufacturers for a wide variety of signal processing and data acquisition applications, including ADC driving, single-ended to differential conversion, IF and Baseband gain blocks, differential buffers and line drivers.
The AD8138 offers versatile functionality with a wide bandwidth of 300 MHz and low harmonic distortion of -94dBc at 5MHz worst harmonic. A proprietary internal feedback architecture provides balanced output gain and phase matching, which suppresses unwanted harmonics. Internal feedback also minimizes any gain error associated with mismatches in the external gain setting resistors.
When driving high performance ADCs, the AD8138 eliminates the need for a transformer, while preserving the low frequency and DC information. Fast overload recovery safeguards sampling accuracy. The AD8138 operates from DC to 300 MHz and interfaces easily with ADI�s high performance 12- to 16-bit family of ADCs, including the AD922x, AD924x, AD9260, AD9042 and AD6640.
"This low distortion ADC driver is a major advancement over single-ended op amps and transformers for differential signal processing" said ADI spokesperson. "This design allows engineers to get the best performance out of their ADCs as well as current-output DACs. The benefits of differential signal processing have long been understood, but until now, no high performance differential amplifiers have been available for linear signal processing. The AD8138 builds on the company's expertise and leadership in high performance analog ICs," Bob Briano said.

If you had been present when I first heard about this product you would probably have described my reaction (being kind to me) as effusive, something that happens only a handful of times a year. This is a dream product for analog engineers; although described as a differential ADC driver, and obviously tuned for that role, this is much more. The part is a differential-input, differential-output amplifier with the unique ability to convert a signal from single-ended input to differential output, or from differential input to single-ended output. In effect, this becomes an electronic transformer or balun, which can still also be used as a differential signal buffer and gain amplifier. With the added, considerable, advantage that the low-frequency performance is not compromised this simplifies conversion problems to the level of "Idiot-Proof."
Given the bandwidth of the AD8138, and the extremely low harmonic distortion that it offers, this product will find its way into all sorts of IF and line driver applications; those uses may well exceed the quantity being used for ADC driving, the latter requiring an analog design engineer in the loop dedicated to truly optimizing a data acquisition system. That is not the norm today.
The AD8138 is in production now in an 8-lead SOIC and will be offered soon in a microSOIC-8; it is priced at $3.75 in 1000-piece lots.

Analog Devices' Active Transformer
By Paul McGoldrick

It is an analog engineer�s dream: To have transformer action in an active device without inductance, cores, weight and cost. Throw in dc-dc and low-frequency performance and the dream is complete. Analog Devices is now feeding that dream with the AD8138 differential op. amp. This is an extremely clever product and will be commercially very successful.

Designed to optimize the drive to a high-performance ADC, the differential input/differential output part has some operating modes which will prove useful for its original purpose and for many other tasks. The amplifier can be used as a fully differential part; or the input can be single-ended with the output differential; or the input can be differential with the output single-ended. While the single-ended to differential mode is the one most expected to be used for driving precision ADCs, the differential to single-ended mode may well be useful at the output of an ADC in any number of applications. There is also a screaming need for a decent high-frequency differential amplifier in all sorts of communications applications and this part fits the bill.

The bandwidth and distortion numbers from the AD8138 are impressive. With a -3 dB bandwidth at 300 MHz (large signal bandwidth is 265 MHz) and a second-harmonic distortion at 20 MHz of -86 dBc the part will be ideal for IF and baseband gain block use in a number of industries. Low-frequency performance and dc information are not compromised in the amplifier, a rather different situation compared to the use of a transformer when driving an ADC, for example.

Operation and Gain

The amplifier, in a differential mode, differs from a standard voltage-feedback op. amp. in requiring two feedback loops, external to the part (see Fig. 1.) The differential-mode gain is given by:

V_{OUT, dm}/V_{DIN, dm} = R_F/R_G

assuming that the values of the resistors are the same for both sides of the input. If there are small mismatches in the resistors that will affect the gain accuracy, but the signals at each output will still be equal and 180� out of phase, forced so by the internal common-mode feedback circuitry.

where,
DIN = differential input
dm = differential mode
cm = common mode

The voltage between the two differential inputs of the part (+IN and -IN) will be nulled when operation is in the linear region, as in an op. amp., and there is also little current flowing into, or from, each input (typically 3.5 µA in differential mode, 0.3 µA in single-ended mode) while the offset voltage is about �1 mV.

Common-mode feedback is internal to the part and common-mode gain is always unity, subject to the matching of the gain and feedback resistors of the same nominal value. The input resistance in differential mode is the sum of the gain resistors while it reduces to approximately 1.33 times the value of one of the resistors in single-ended to differential mode.

Driving an ADC

Configuring the AD8138 to drive an ADC is straightforward. The distortion of the majority of ADCs is minimal when the parts are being driven differentially. Because the gain of the driving amplifier is fixed at unity, however, any gain adjustments to optimize the resolution of the ADC should be completed prior to the buffer. The example given (see Fig. 2) is of an AD8138 being used to drive the 12-bit 40-Msample/s AD9224.

The bipolar 50-W source, with its parallel termination of 50 W, is matched by the lower-side gain resistor of 525 W compared to the 500 W of the upper side. The AD8138 is run with �5 V supplies to give the best distortion performance, while the output pins drive 50-W resistors to buffer the part from the switched-capacitor effects present on the ADC�s inputs.

The SENSE pin of the ADC is shorted to the AVSS pin which sets the full-scale analog input to 4 V, where harmonic distortion from the part is minimized. The CML pin of the ADC is coupled to the VOCM pin of the AD8138, putting that point at half the ADC supply voltage, at 2.5 V. With a 4-V signal from the generator each output from the AD8138 would swing from 1.5 to 3.5 V.

Other Performance

The 0.1-dB flatness of the AD8138 extends to 34 MHz while the input noise voltage is typically 5 nV/rtHz at 10 MHz. The slew rate is 1000 V/µs. The supply range is from �1.5 V to �5 V, while the quiescent current is typically 20 mA.

The AD8138 is in production in Analog Devices' XFCB bipolar process in an SOIC-8 and will also be available in a µSOIC-8. It is priced at $3.75 in 1000-piece lots.

Analog Devices, Inc. Ray Stata Technology Center, 804 Woburn Street, Wilmington, MA 01887. Tel: (781) 937-1428. http://www.analog.com/highspeedamps.

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