Precision Function Generator: MAX038 goes SMD （1）

Published:2011/8/7 22:31:00 Author:Li xiao na From:SeekIC

By Klaus-Jiirgen Thiesler



Although surface-mount parts are not the easiest to handle, the reward in test and measurement equipment is increased accuracy.

Integrated precision function generator type MAX038 is now an old faithful with a history of ten years. In the June 1995 issue we described a full-blown function generator built from standard size parts and using what is now considered a large PCB. The project was hugely successful and PCB sales reached several thousands. In 1995, the use of SMDs in Elektor circuits was more or less prohibited because the advantages of a smaller board did not outweigh the disadvantages of problematic supply and mounting methods.

Today there’s no way you can avoid SMDs anymore, however we generally do not use them unless they serve a good purpose, or if the component we wish to use is simply not or no longer available in ’leaded’ form. Other compelling reasons to use SMS are the ongFoing miniaturization of circuits and near elimination of stray capacitance and inductance (particularly in compact RF gear).

In some case, however, SMDs simply serve advancing technology — if you build the function generator from SMD instead of leaded parts and ensure low-inductance (i.e., shown) PCB tracks then the instruments’ technical specifications as well as its frequency range can be improved dramatically.

A digital frequency range switch in combination with multilayer ceramic capacitors (MLCC) in SMD technology not only improves the general performance of the MAX038 chip but also extend its frequency downwards to below 0.1 Hz and upwards to over 20 MHz. Such improvements we feel are worth just that extra effort in building up the circuit.

Two easily realized design aspects will help to unlock the full potential of the MAX038:

- an optimized PCB layout with the shortest possible copper tracks to keep stray inductance and capacitance to a minimum;
- SMD parts are marked by lower temperature coefficients and closer tolerances.

The circuit and PCB layout presented here do not represent a complete construction project. Rather, the publication aims at providing a few useful tips when you really want to juice the MAX038 for maximum performance. Hence Mr. Thiesler’s contribution fills this month’s Start Here pages.

Range switch

Instead of the mechanical range switch we propose an electronic CMOS analogue switch type MAX4638ESE, an 8-1 multiplexer that, depending on its control, connects the various frequency-determining capacitors to the internal oscillator. In the highest frequency range (20 MHz), it is essential for capacitor C8 to be connected directly to the MAX038 — and not by way of the multiplexer. Using the SMD stuffing in SO 16 and 0815 enclosures this can be realized without problems. The ground side of C8 has to be connected directly to pin 6 (GND) — the other ground pins are unsuitable for that purpose.

The switching currents through the frequency determining capacitors are really low and the oscillator voltage (COSC) is just-1 V (at a supply of ±5 V). This makes the selection of the CMOS switch rather easy. Much more critical however is the PCB layout; 1 cm of 1-mm wide copper track representing an inductance of 7 nH. Long wires cause large stray capacitances and that is where SMDs are clearly superior to their leaded counterparts. As a bonus, SMDs are manufactured to closer tolerance specifications.

Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/07/Precision_Function_Generator__MAX038_goes_SMD_（1）.html

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