VGA chip wanted.
Gain range : -20db~40db
BW : 125MHz(0~125M)
AD8330 is a DC to 150 MHz Variable Gain Amplifier that will be suited for your application. It has two gain controls namely Linear-in-dB Gain control (VDBS) and Gain Magnitude Control (VMAG). When the VMAG pin is unconnected it will give a default gain range of 0 dB to 50 dB. However, by applying ~15 mV to 5 V to this control pin will either lower the gain by 30 dB or raise it by 20 dB. This will now extend the gain span to 100 dB (-30 dB to 70 dB). Please refer to the "Theory of Operation" section for further details regarding this feature.
May I know your design considerations and application information/ requirements in terms of device performance? What is your application? Can you share it with us?
Hi Cao san,
I thought I could contribute a bit to this conversation. The AD8330 suggested by Jino is one of our most popular parts and enjoys a wide variety of applications. Fortunately in this performance range you have a lot of options. You may want to consider not only the bandwidth and gain range but tolerances across the bandwidth, single-ended vs. differential signal path, circuit complexity, usage of preamplifier, etc.
For example, the AD8330 does indeed offer a very wide dynamic range, however the part consists of two VGAs with 100dB total dynamic range (-30 tp 70 dB total. Notice however that the first VGA is linear in dB, the following is linear in magnitude. This requires two different gain control circuits as the linear in dB is 1.5V, the linear in magnitude is 0 to 5V. Also the upper bandwidth limit is 150MHz for 3dB, typically. If gain is critical at 125MHz, you would be looking at about 25% range in frequency for the same gain because the upper BW limit has larger range in frequency than at lower frequencies. I usually recommend using an amplifier of 3x to 10x larger than the gained required at the upper end of the desired frequency spectrum, but this may not be practical with the specs you provided. There is a tradeoff between gain range and BW.
Should you like the AD8330, and you also need good dc performance you will have to consider the input requirement of the AD8330. This type of VGA requires a voltage source input drive to operate properly, preferably differential. If your drive happens to be single-ended, the most straightforward approach would be to use a SE to differential converter typically used for driving ADCs.
Another approach would be to connect two VGAs in series, where one can drive the two with the same drive source. Since you've specified dc as your lowest frequency of interest, you might consider two AD8337s connected in series. Each has a nominal BW of 280MHz, but as before, the upper limit would be subject to a greater frequency tolerance.
Lots to think about, best of luck.
two AD8337s seem not to meet my gain request.I need gain range -20db ~ 40db(60db total),but the two AD8337s in serial only have
gain range 0db ~ 48db(48db total).Right?The attachment is the analog part of my design.I want to use ADI chips for all,but I am not familiar with the analog circuit.Could you give me some advice.The another question is does ADI has a PLL with jitter clean and clock distribution functions and 250MHz and 500KHz can be generated when the input is 125MHz.
The attachment is a solution for my design.Would you give me some advice for that?
Hello Cao San,
Your part 2 attachment is a high level block diagram and appears the same as part 3. I can't really tell what you're trying to do but the AD8330 isn't very accurate at DC. It was actually designed for ac coupling, some people use it for dc on certain special applications such as an AGC amplifier where the only gain control connections and output signals are dc.
The devil is in the details, but if you really need dc- 125MHz performance the AD8330 may not meet your needs. If you can use ac coupling there are a lot of candidates.
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