ADC Drivers Combine Low Noise, Low Distortion and Ultra-Low Power Consumption

“We chose to use the ADA4932 because it is the only amplifier that allowed us to hit bandwidth and distortion targets within our power budget,” said Matt Ernst, hardware engineer at National Instruments, a leading provider of automated test equipment and virtual instrumentation software. “This amplifier has great specifications for both performance aspects, especially for the amount of power it uses.”

The ADA4932 ADC driver combines the low noise and distortion levels required to drive high-performance 10-bit to 16-bit ADCs found in today’s power-sensitive medical-imaging, wireless-infrastructure and instrumentation applications, including ultrasound scanners and picocell and femtocell wireless base stations. With fixed-gain options of 1x, 2x, and 3x, ADI has optimised the ADA4950 for use in smaller designs. In addition to their low supply current, these new additions to ADI’s low-power ADC-driver family achieve 95-dB SFDR (spurious-free dynamic range) and true 14-bit performance at 20 MHz, as well as true 12-bit performance out to 50 MHz.

The ADA4932 and ADA4950 can operate as single-ended-to-differential or differential-to-differential amplifiers. When performing single-ended-to-differential conversion, the amplifiers require no additional signal conditioning, resulting in better ADC performance with no added components. ADCs that work well with the ADA4932 and ADA4950 include ADI’s high-speed converters, such as:

- AD9259 quad 14-bit, 50 MSPS ADC
- AD9244 14-bit, 65 MSPS IF sampling ADC
- AD9226 12-bit, 65 MSPS ADC
- AD9229 quad 12-bit, 65 MSPS ADC
- AD9863 12-bit MxFE integrated converter

ADA4932 ADC Driver: Integration, Simplification

The ADA4932 integrates the functionality of a number of discrete components to simplify design tasks for engineers while saving board space. Its output common-mode voltage is user-adjustable by means of an internal common-mode feedback loop, allowing the ADA4932 output to match the input of the ADC. The internal feedback loop also provides exceptional output balance as well as suppression of even-order harmonic distortion products. With the ADA4932, differential gain configurations are easily realised with a simple four-resistor external feedback network that determines the amplifier’s closed-loop gain. The ADA4932 is fabricated using ADI’s proprietary SiGe (silicon-germanium) complementary bipolar process, enabling it to achieve low distortion and noise while operating at low power consumption. The low offset and excellent dynamic performance of the ADA4932 make it well suited for a wide variety of data acquisition and signal processing applications.

Manufactured on ADI’s proprietary XFCB (extra-fast complementary bipolar) process, the ADA4950 has a −3dB bandwidth of 750 MHz and delivers a differential signal with very low harmonic distortion. The ADA4950 has an internal resistor network that users can configure for gains of 1, 2, and 3 in an otherwise fixed architecture. The ADA4950 has an internal common-mode feedback loop that provides a balanced output with gain and phase matching and suppresses even-order harmonics. The internal feedback circuit also minimizes any balance error that would be associated with mismatches in the gain-setting resistors. The ADA4950 driver’s differential output helps balance the input to differential ADCs, maximising the performance of the ADC.

The ADA4950 eliminates the need for a transformer when driving high-performance ADCs, preserving the source signal’s low-frequency and DC information. The common-mode level of the differential output is adjustable by applying a voltage on the VOCM pin, which level-shifts the input signal for driving single-supply ADCs. The amplifier’s fast overload recovery preserves sampling accuracy. The distortion performance of the ADA4950 ADC driver is attractive for communication systems, while its wide bandwidth and high IP3 also make it desirable for use as a gain block in IF and baseband signal chains.