Simple Circuits Add to Versatility of the AD9834 Direct Digital Synthesizer IC

The little AD9834 Direct Digital Synthesizer (DDS) made by Analog Devices is a powerhouse of a design that is found in all sorts of products from Radios to Power Supplies. It is the “NE555” of DDS chips in it's popularity [1].

The AD9834 consists of a 28 bit programmable DDS Core and a 10 Bit Current Output DAC. The Nominal DAC output current is 0 to 3 mA. This current can conveniently be output directly into a 200 Ohm load to generate a 0 to 600 mV output voltage (See figure 1).

Figure 1 – Standard output circuit configuration for AD9834 with a FSADJ resistor of 6.81k provides a fixed 0 to 600 mV output.

Note: The following circuits are simplified and do not show power supply connections or proper bypassing. Please refer to the parts specific data sheets for complete usage information.

While 0 to 600 mV may be useful in many applications it is not particular useful if the output voltage needs to be user adjustable or bipolar. This is especially true when the DDS is used like a function generator where the end user needs an adjustable amplitude.

Adjustable DAC Full Scale Current

The first approach to adding an adjustable output to the AD934 is to attack the DAC current setting resistor. This resistor is nominally 6.81k for a DAC current of 0 to 3mA. The voltage at pin 1 (FSADJ) is nominally 1.15 Volts and this generates a current in the 6.81k resistor of 0.1689 mA (1.15/6810). This current gets scaled by 18 times internally in the AD9834 to get to the final 3mA DAC Full Scale Current.

The internal design of this circuit lends itself to controlling the DAC full Scale current over a reasonable range and this can be a useful and inexpensive way to get an analog or digital adjustment on the DDS output voltage.

As shown in Figure 2, a simple precision OPAMP from Linear Technology [2] used in a scaling circuit has been added to the DDS to control the DDS output voltage over a 4:1 range. Maximum DDS output voltage for this circuit is achieved for a 0 Volt control input.

A potentiometer or any 0 to 5V DAC output can be use as the 0 to 5V input to allow complete digital control of the DDS output voltage.

Figure 2 – A simple OPAMP circuit added to the AD9834 can give the AD9834 a 4:1 output Adjustment Range for a 0 to 5 Volt input signal. The Input signal could be a Potentiometer or from a 0 to 5V DAC.
 
The limiting factor in the maximum achievable adjustment range of the circuit in Figure 2 is the AC performance of the DDS DAC. While the output can be adjusted down from its maximum value the feedthrough glitches from the DAC switches will remain the same and the linearity of the DAC will suffer at lower output levels. Note also, that any excess noise on the 0 to 5V control voltage will additionally cause AM Modulation on the DDS output so add filtering as may be required by your application.

I have used this circuit in Figure 2 for a 4:1 adjustability with decent results. Your mileage may vary, so be sure to check the AC parameters that are important in your specific application.

Multiplying DAC on the DDS Output

For the ultimate in digital adjustability a Multiplying DAC  (MDAC) can be used at the output of the DDS to get 2^14 (16384:1) or better than 80 dB of adjustment range.

The AD5453 family from Analog Devices is a very high bandwidth Multiplying DAC [3] that comes in 8, 10, 12 and 14 bit resolutions. It takes in a AC reference voltage and outputs a Current that is scaled by a Digital Control Word.

Most MDAC's have a very low -3 dB bandwidth on the order of 20 kHz, the High Speed AD5453 when used with a suitable OPAMP output has a -3 dB bandwidth of 10 MHz or better. The maximum attenuation (or how low you can control the output) is flat to 300 kHz at 14 bits, rising to 1MHz at 12 bits, 3MHz at 10 bits finally rising to 10 MHz at 8 bits.

Figure 3 – Combining a high speed AD5453 MDAC and a LT1087 Dual OPAMP allows very complete control of the AD9834 DDS output.

Note: The 10 uF capacitor sets the low frequency roll off. With the 10 uF value shown the low frequency, -3 dB point is below 2 Hz. (The input resistance of the AD5453 VREF Pin is 7k Ohms minimum).

Note: The 1.5pF capacitors should be adjusted in the final circuit for maximum output flatness over frequency.

The circuit of Figure 3 provides a +/- 5 Volt output with low distortion to 1 MHz and provides 80 dB plus of output voltage adjustment range. Additionally the output can be offset from -5V to +5V with the addition of an offset adjustment control via a low cost DAC or POT (Offset Adjust Input).

DDS Output Control At Even Higher Frequencies

If you need to operate the AD9834 at even higher frequencies, closer to the maximum specified fundamental output of 37.5 MHz, or even operating in “Super Nyquist” mode [4] you should look at a 50 Ohm CMOS RF Attenuator like those manufactured by Peregrine Semiconductor [5]. The PE43711 has a frequency range down to 9 kHz and 31 dB of control with 0.25 dB steps all the way to 6 GHz. At higher frequencies you will probably be designing around 50 ohm circuit impedance's anyway so this should not be much of an issue. Multiple PE43711's can be connected in series to get more attenuation in 31 dB chunks.

Note: At lower RF frequencies, less than about 50 MHz, CMOS, SiGe and Silicon based IC's are preferred to GaAs IC's. This is because the GaAs IC's typically have worse harmonic distortion (Especially very bad 2nd harmonic distortion) at low RF frequencies.

References:

[1] The NE555 timer is arguably the most popular linear IC of all time.

[2] Linear Technology LT1677 Precision and LT1087 High Speed OPAMPS are manufactured by Linear Technology Inc. Now a part of Analog Devices.

[3] Analog Devices Application Note: “Multiplying DACs Excel at Handling AC Signals”.

[4] Super Nyquist Mode, See: Analog Devices Application Note AN-939

[5] Peregrine Semiconductor, Inc

Article By: Steve Hageman www.AnalogHome.com 
 
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