An internal common-mode feedback loop allows the user to independently adjust the output common-mode level to match the input common-mode voltage of the ADC as well as achieve exceptional output balance and suppression of even-order harmonics. Other key specs include its 2.3-nV/Hz input voltage noise, 1.4-GHz, –3-dB bandwidth (G=+2), and 5000-V/µs slew rate.
For quadrature demodulation schemes, Linear recently announced dual versions of its LTC6400/LTC6401 differential driver. The dual LTC6420-20 and LTC6421-20 add guaranteed ±0.25-dB gain matching and typical ±0.1° phase matching to the noise and distortion performance of the original singles. Channel separation is 80 dB at 100 MHz. The LTC6420-20 provides a fixed gain of 20 dB with –84-dBc thirdorder intermodulation distortion (IMD3) at 100-MHz input frequency. Input voltage noise is 2.2 nV/Hz, including internal gain-setting resistors.
These amplifiers achieve their distortion and noise specs on a single 3-V supply and have rail-to-rail (R-R) output swing. Thus, the dual ADCs and amplifiers are able to share the same voltage supply in many applications. These drivers can drive an ADC directly without any external output impedance matching and convert single-ended inputs into differential outputs. The LTC6420-20 operates from dc to 300 MHz. The lower-power/lower-priced LTC6421-20 operates to 140 MHz.
In the August 14 issue, I wrote about Maxim’s MAX2065 fully programmable analog and digital IF/RF variable-gain amplifier, which is a nice example of advanced design (see “RF/IF VGA Chip Does It All,", ED Online 19427). Maxim packed a linearly controlled, 31-dB voltage-variable attenuator; a 31-dB digital step-attenuator; a 22-dB gain driver amplifier; an 8-bit, control digital-to-analog converter (DAC); and a simple SPI-compatible interface into one chip.
The point was to come up with a virtual Swiss Army knife for applications in GSM/EDGE, CDMA, WCDMA, LTE, and WiMAX receivers. Designers can use the MAX2065 as either an IF or RF allpurpose variable gain amplifier (VGA), interfacing directly with 50- systems operating anywhere between 50 and 1000 MHz. Each of the three independent RF stages has its own RF input and RF output, so the chip can be configured to optimize either noise figure or linearity (Fig. 3).
Another approach to “high performance” is to take an intransigent problem that’s been bugging circuit designers for decades and solve it for them. That’s what Texas Instruments did this year with some of its R-R op amps. The first was the OPA365 in January, and the latest was the OPA369 in June. The latter does it with less power.
The basic problem is that the input stage for most R-R amps consists of a p- and n-channel differential pair in which offset voltage depends on the common-mode input voltage. In other words, there’s a nonlinearity as the input signal passes through the crossover point between the two devices, and that limits the amplifier’s total harmonic distortion (Fig. 4). To combat the problem, TI’s chip designers use a charge pump that boosts the positive supply by about 2 V to supply the input tail current for a pMOS differential pair on the input.
In the OPA369, this approach delivers an offset voltage of 750 µV over the entire R-R input range and a common-mode rejection ratio (CMRR) of 100 dB minimum, maximizing the usable input dynamic range for low-supply-voltage applications. Other features include an input voltage noise density of 120 nV/Hz, a 12-kHz gain-bandwidth, an input bias current of 50 pA maximum, a voltage offset drift of 1.75 µV/°C (max), a power-supply rejection ratio (PSRR) of 94 dB, and 1/f noise of 3.6 µV p-p from 0.1 to 10 Hz.
The OPA369 offers the precision, low power, and small packaging required in a wide variety of applications. These range from portable medical devices (glucose meters, oxygen metering), portable instrumentation (gas detection/monitoring), and sensor signal conditioning to fitness-related portable consumer equipment, cellular phones, and handheld test equipment.