acquisitionZONE Products for the week of December 12, 2011

Texas Instruments Says…

INA149: Difference Amplifier For ±275 V Common-Mode Applications

Texas Instruments Incorporated (TI) introduced the industry’s most accurate difference amplifier for high common-mode-voltage applications up to +275 V. With a best-in-class common-mode rejection ratio (CMRR) of 100 dB, the INA149 increases overall measurement accuracy by 2x compared to the competition and is the first difference amplifier with guaranteed 90 dB CMRR performance in harsh, high temperature environments up to 125 degrees C. The amplifier also lowers initial gain error by 33 percent compared to the competition while providing twice the slew rate to increase response time and overall system performance.

“The INA149 combines an extended common mode range with industry-leading CMRR for the highest precision measurement of low differential signals in industrial designs. For our customers, this means higher performing, more accurate products,” said Steve Anderson, senior vice president of TI’s High Performance Analog business. “The device also eliminates the need for multiple additional isolators or multiple power supplies in high common mode voltage applications to reduce component count, improve reliability and simplify system design.”

The INA149 can be used in a variety of high common-mode-voltage applications, such as high-voltage current sensing, battery cell voltage monitoring in photovoltaic, telecom, electric vehicles, and alternative energy applications; power-supply current monitoring; and motor control.

EN-Genius Says…

The INA149 is TI’s reaction to competition. The world of unity gain high common-mode voltage amplifiers was led by the company’s INA117, and then Analog Devices came along with the AD629. TI have now responded with the INA149 both in specifications and pricing.

For the most part the INA149 betters the AD629 (of which there are two grades: A and a tighter B) with ±275 V common-mode voltage range (as opposed to ±270 V), a maximum gain error of 0.02% (cf 0.03%), a 5 V/µs slew rate (cf 2.1 V/µs), and a maximum quiescent current of 900 µA (cf 1 mA). The maximum gain error drifts are similar at 10ppm/ºC, as are bandwidth at 500 kHz (32 kHz full-power bandwidth is much better), and input protection voltages at ±500 V for both common-mode and differential.

The INA149 offers a maximum offset voltage drift of 15 µV/ºC, placing it midway between the AD629A at 20 µV/ºC and the AD629B at 10 µV/ºC but the maximum offset voltage itself is poorer at 1100 µV (cf 500 µV), and the maximum gain nonlinearity appears worse at 0.001% FSR compared to 3ppm. Output swings appear about the same percentage although quoted at different rail voltages with the INA149 offering ±13.5 V on ±15 V rails and the AD629 offering ±10 V on ±12 V rails. The available operating voltage range on the INA149 is also extended from ±2.0 V to ±18 V while it is limited to ±2.5 V at the lower end with the ADI product.

But the big improvement in the specifications is in the CMRR – which is what these difference amplifiers are all about. Across the range the TI product is at least 15 dB better offering a minimum of 90 dB across the temperature range of -40ºC to +125ºC, with a typical 100 dB out to 1 kHz.
The INA149 offers an output noise voltage of 20 µV pk-pk out to 10 Hz and 550 nV/rtHz at 10 kHz. The short-circuit current is typically ±25 mA and the part can drive a 10 nF capacitative load.

These amplifiers are for use in applications where galvanic isolation is not needed and they save the expense of isolation amplifiers (and their power supplies) and their more limited performance. Care must be taken to recognize the effects of the input resistance of the devices (typically 200 kΩ common-mode) and that the common-mode voltage range is severely limited at lower rail voltages where the input dividers cannot bring higher common-mode voltages down to the input range of the amplifier itself. The part can be used with a single rail, if required, and reference inputs allow for the transfer characteristic to be modified from a straightforward unity-gain difference amplifier.

With the INA149 TI are taking back the lead in the high common-mode voltage difference amplifier battle and will maintain it for quite some time. They are also pricing the new part about twenty cents lower than the now poorer performing competition. Both are recipes for high volume success.

The INA149 is in production in SOIC-8 and is priced at $2.70 in 1000-piece lots. An evaluation module and TINA-TI SPICE model are available.

Data Sheet