LMH6503: LMH6503

Hi Fulvio,

this scheme is intended to null the offset voltages. As voltage controlled gain stages usually react very sensitively to offset errors, I don't think that the nulling scheme can be used to create such a high imbalance. This would be just the opposite of what one would want to achieve with the nulling scheme.

Kai

Hello Fulvio,

  Page 19, Figure 61 of the LMH6503 datasheet demonstrates how to nullify the offset voltage by adjusting resistance via trim pot to adjust the voltage applied to the input of the amplifier in order to "cancel" the amount and the lower the amount of output offset voltage of the device. Figure 37 to Figure 43 are important figures that go along with this section. 

  As Kai pointed out, this is the opposite of what you would want for your design. You can apply a DC bias at the non-inverting input of the amplifier similar to biasing a standard amplifier as shown below or via voltage divider (be careful with bias current of the amplifier).

   Also, LMH6503 can not be powered with a +/-12V power supply. The maximum Vs of the device is 12V. Did you mean to set configuration to 12V single supply with 5V DC common-mode voltage? 

Thank you,
Sima 

Good to have you here, Michael

Thanks Kai, sometimes these questions overlap the parts I worked on, so easy to fill in the details. The original CLC520 and CLC522 consumed a lot of my time in their release - made a pretty detailed effort at a new noise model for this type of part, that sits inside the old CLC522 or CLC520 datasheet, but I don't think anyone brought that forward in the later parts - the gain control gave an interesting peak in noise (flatband) around 6dB attenuation I vaguely recall. Countless hours of taking data and postulating an equivalent model there. 

And of course when we bumped up against parasitic oscillations, those things you don't forget - the original parts had a 1.2GHz self oscillation in the input stage of the output op amp if you just grounded that V+ input - hence, the datasheets I did showed a 20ohm external to ground there to deque that emmitter follower oscillation. 

Hello, thanks Sima for the reply.
Ok about the offset, but in the datasheet on page 3 it is specified: "Absolute Maximum Ratings Supply Voltages (V + - V−) = 12.6V", therefore the dual power supply applicable to the LMH6503 can also be (V + - V−) = 12 V, which is less than the Absolute Maximum Ratings Supply Voltages (V + - V−) = 12.6V. In fact, in the case of power supply not exceeding V + - V− = 5V, the value should be: "Absolute Maximum Ratings Supply Voltages (V + - V−) = 5.6V or 6V".

Thank you,

Hello Fulvio,

  That is correct, (V+ - V-) must be less than 12.6V to avoid damaging the device. The original question listed power supplies at +/-12V (Vs+ = 12V and Vs- at -12V) which would equal to a differential supply voltage of 24V. 

Thank you,
Sima 

Ok, sorry, I had interpreted (V + - V-) as (V + / V-) dual power. Is there a chip equivalent to the LMH6503 (LMH6503MA / NOPB package since I already made the PCB) that can be powered at + 12V / -12V or + 15V / -15V?

Thank You

there is not, you should change your supplies - if you need more swing, you will need another postamplifier on +/-15V supplies, here is a short selection table of single CFA higher voltage options, 

Hello Fulvio,

  Here are additional high supply PGA/VGAs: https://www.ti.com/amplifier-circuit/pga-vga/products.html#p358max=24;36

Thank you,
Sima