Dear ladies and gentlemen,

in our project on silicon quantum computation we require ultra low

noise operational amplifiers to measure current changes of pA on nA

Signals. (Thus input impedances of order 10^12 ohm)

I checked your website and found the ADA4817 with nV/sqrt(Hz) voltage

noise and pA/sqrt(Hz) current noise. Our sample resistance of

25-100kOhm necessitates 1-100 fA/sqrt(Hz) and 1-50 nV/sqrt(Hz).

We have 2 I/V converter applications in mind:

1. "fast&high IV gain measurements"(1-10 kHz range) - for pulses and

Voltage noise insensitive measurements high ohmic feedback Resistor

(1-100GOhm) current noise corner frequency <1-10 Hz voltage noise cf

<1-10kHz gain bandwidth product (Gain 1) >100MHz-1GHz

2."slow&medium IV gain measurements"(1mHz- 1kHz) - for small signals,

long averaging and voltage noise sensitive measurements medium ohmic

feedback resistor (100MOhm-1GOhm) current&voltage noise corner

frequency <10Hz (voltage corner if possible <1Hz) gain 1 bandwidth

>10MHz

The noise level was specified @100kHz. How high are these levels below? Is the

ADA4817 fit for any of the cases or can you recommend an alternative?

Further “nice to have” specs are a TO99 8pin packaging factor, possibility of

+/-15V source operation and an pin for external offset compensation.

Thank you for your help.

Hi qutechie,

Thank you considering ADI parts on your project. Unfortunately, ADA4817 could not meet your requirements on low frequencies since the input voltage noise at 10Hz is between 100 to 200 nV/sqrt(Hz) and at 100Hz its noise density is 40nV/sqrt(Hz). Please see the graph below (Figure 30 page 11 of the datasheet).

ADA4627-1/ADA4637-1 would be a good replacement for that part. It can operate at +/-15V and has a current noise density level of 2.5fA/sqrt(Hz). It also has a low voltage noise level of 16.5nV/sqrt(Hz) at 10Hz and 4.8nV/sqrt(Hz) at 10kHz, which is within the range of your specified requirements. Please see the picture below for the graph.

However, it does not come out on 8-pin T099 package. It only comes in LFCSP and SOIC packages.

I hope this helps, just feel free to ask questions if you need help.

Thanks,

Roel