Operation of AD8375 and AD8376 Digitally Controlled VGAs at Low Frequency

Q. The frequency response plots in the AD8376 and AD8376 DGA data sheets show roll-off at low frequency. Is there any way around this? ---------------------------------------------------------------------------------------------------------------- A. The AD8375 uses an open-collector output structure that requires dc bias through an external bias network. Typically, choke inductors are used to provide bias to the open-collector outputs. Choke inductors work well at signal frequencies where the impedance of the choke is substantially larger than the target ac load impedance. At lower frequencies, the impedance of these inductors decreases and causes the gain to decrease. In broadband applications, it may not be possible to find large enough choke inductors that offer enough reactance at the lowest frequency of interest while offering a high enough self resonant frequency (SRF) to support the maximum bandwidth available from the device. The circuit below can be used when frequency response below 10 MHz is desired. This circuit replaces the bias chokes with bias resistors. The bias resistor has the disadvantage of a greater IR drop, and requires a supply rail that is several volts above the local 5 V supply used to power the device. Additionally, it is necessary to account for the ac loading effect of the bias resistors when designing the output interface. Whereas the gain of the AD8375 is load dependent, R L , in parallel with R1 + R2, should equal the optimum 150 Ω target load impedance to provide the expected ac performance depicted in the data sheet. Additionally, to ensure good output balance and even-order distortion performance, it is essential that R1 = R2. This circuit is discussed in more detail on page 14 of the Rev 0 data sheet.

What are the precautions to prevent latch-up on the AD8375?

Q. What are the precautions to prevent latch-up on the AD8375? -------------------------------------------------------------------------------------------- A. The outputs of the AD8376 are open collectors that need to be pulled up to the positive supply. This can be done via chokes to VDD or resistors. When using resistors the voltage drop across the resistors needs to be accounted for therefore the higher pull up supply. The circuit showing the 9V is configured correctly. The 9V minus the drop across the resistor results in the output of the AD8376 sitting at 5V. The chokes pulled to 5V does the same exact thing. For wide band applications the resistors are the best option because you’re not bandwidth limited by the parasitic capacitance and inductance of the chokes. However leaving the 9V supply on while the part is powered down will cause the output pins to go to 9V. This will turn on the esd protection diodes as well as exceed the break down voltage of the output transistors. So it's necessary to ensure that the 9V supply is only on when the device is powered up. One option is to install 6.5 volt clamps from the outputs to ground. So when the part is powered down, the current would go to about 50ma total.