acquisitionZONE Products for the week of July 16, 2012
Intersil Corporation Says…
ISL28617: 40 V, Low Noise, Precision Inst Amp with Integrated ADC Driver
Features Excellent Gain Accuracy, Highest Common-Mode Rejection Ratio
Intersil Corporation announced a new 40V, low noise instrumentation amplifier that is the industry’s first precision instrumentation amplifier with an integrated analog-to-digital converter (ADC) level shifter and driver.
The ISL28617 incorporates a differential output to easily interface to today’s highest performance differential input ADCs. It offers excellent gain accuracy and gain temperature coefficient over a temperature range of -40°C to 125°C, with the industry’s highest common mode rejection ratio (CMRR) in all gain configurations. Noise measures only 8.6nV/√Hz @ 1kHz and 85nVrms from 0.1Hz to 10Hz, and the ISL28617 can provide both high gain and attenuation, setting it apart from competing instrumentation amplifiers which typically are limited to gain only configurations. The ISL28617 is ideal for instrumentation, industrial, motor control and medical systems.
The ISL 28617 is a differential input and differential output instrumentation amplifier that is provided with an internal ADC driver. The design parameters were to produce an output with a significant voltage rail that could be limited on swing to prevent overdrive and that was also close to rail-to-rail; gain accuracy was to be limited to controllable external components; provide any gain, positive or negative; maintain a very high CMRR; level shift to the following ADC with a common external reference for both products.
The part achieves this in abundance. Gain is selected using two external resistors, one as RIN and the other as RFB, with gain the ratio of the latter to the former. With a 121 kΩ resistor for feedback gains from unity to 1000 (input resistances 121 kΩ to 121 Ω) frequency responses are between 100 kHz and 1 MHz – more than adequate for the intended applications. Both the feedback and input resistor connections are also provided with Kelvin sense terminals.
With a typical three amplifier design the input offset is a typical ±30 µV and ±0.2 nA with an input resistor drive current of 100 µA and a common-mode input resistance of 80 GΩ. The common-mode input voltage can be in a range within 3 V of each rail. The device can be operated from rails extending from ±4 V to ±20 V (although the part could be used on single rails). The outputs can extend to within 150 mV of each rail. The input noise voltage density is a good 8.6 nV/rtHz while the input noise current density is a typical 150 fA/rtHz. For best noise in operation Intersil recommend Vishay PG Z-1 bulk metal foil resistors for the gain-setting passives.
Both the positive and negative PSRR depend on the feedback resistance, gain and frequency. Within the probably used 100 Hz range negative PSRR is about 120 dB for gains higher than unity (with RFB of 30.1 kΩ) and positive PSRRs are also about 120 dB for gains higher than 10. CMRR are also about 120 dB for unity gains increasing to about 150 dB at a gain of 100.
The number of vendors who are capable of producing truly precision analog products is now limited to about three companies: despite claims made by others. Intersil is one of that trio of top-end suppliers. The fact is, however, that the more precision a part the more difficult it is, in general, to use it to its full potential. The ISL28617 is capable of a lot but it is going to keep Intersil’s application engineers busy. The part is also underpriced, by at least two dollars.
The ISL28617 is an amazing part and will find use as the sense amplifier/ADC driver in biomedical applications, t&m, and precision weight and pressure scales. It will be paired in many cases with an Intersil high-precision 24-bit ADC such as the ISL26134.
In production in TSSOP-24 the ISL28617 is priced at $2.85 in 1000 piece lots. An evaluation board is available.