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# LMH6551Q-Q1: Worst output voltage swing and condition

Part Number: LMH6551Q-Q1

Hi,

Our customer is considering to use LMH6551Q-Q1 as a preamplifier of the ADC.

They are worried whether accuracy in the minimum output swing of LMH6551Q-Q1 can be satisfied.

It  is assumed that Vs is 3.3V and Vcm is 1.65V.

The minimum output voltage swing is 0.75 V in the data sheet.

How is the minimum in all temperature ranges?

And, I want to know the condition that becomes worst.

They would like to verify using spice, but is this worst case reflected in the PSPICE model provided in web?

Best regards,

Tomoaki Yoshida

### 7 Replies

• Hi Tomoaki,

The minimum output voltage swing across all temperature ranges should be around +/-0.65V single-ended peak to peak on Vs=3.3V supply and Vcm = 1.65V.

I don't think the PSPICE model provided on the web can reflect the worst case minimum output swing in the actual device.

Best Regards,

Rohit

• In reply to Rohit Bhat:

Hi Rohit-san,

LMH6551Q-Q1 is assumed to be used as a preamplifier amplifier of ADC.

The input common-mode voltage of the ADC is 0.95 V and the input full scale is 1 Vpp with single end.

And the maximum input voltage rating is limited to 2.1V.

#1

What is the maximum of the output swing voltage all over the temperature on Vs=3.3V and Vcm=1.65V?

#2

The output swing voltage should change according to the Vs voltage.

Do you know the Vs condition where the minimum swing voltage does not exceed 1 Vpp and the maximum is less than 2.1 V?

The ambient temperature is assumed to be -40 ° C to 105 ° C.

#3

Is there a returning operation waveform from overdrive recovery performance like the below figure?

Best regards,

Tomoaki

• In reply to Rohit Bhat:

Hi Rohit-san,

It is assumed that the LMH6551Q is used as the pre-amplifier of the ADC.

The single end input full-scale voltage of the ADC is 1Vpp on Vcm=0.95V.

And the absolute maximum rating of the input voltage is 2.1V.

#1

What is the maximum of the output swing voltage over the entire temperature range?

The ambient temperature is assumed to105deg from -40deg.

#2

Although the output swing voltage depends on Vs, do you know the condition that the minimum output swing voltage is as large as possible without exceeding 1 Vpp and the maximum does not exceed 2.1V?

#3

Is there a waveform of overdrive recovery performance as shown in the following graph?

Best regards,

Tomoaki

• In reply to Tomoaki Yoshida:

Hi Tomoaki-san,

1. The maximum of the output swing voltage over the entire temperature range could be +/-0.95V single-ended peak-to-peak on single 3.3V and 1.65V output common mode (Vocm). This specification is basically difference of the two outputs which is [(Vo+)-(Vo-)].

2. Usually, the Vce(sat) of the output transistors decrease with increasing temperature. So, I would think the maximum output swing voltage is possible at the higher temperatures. Is this what you were asking?

3. I don't think we have an overdrive recovery performance as you have shown. However, if you are looking at the maximum and minimum Vout, then you can take a look at Figure 17 and 18 in the datasheet. These curves are for +/-5V supply, but you can extrapolate them for single +3.3V supply or +/-1.65V supply. Basically subtract 1.2V from either rails, or single-ended max Vout = (Vs+) - 1.2V and single-ended min Vout = (Vs-) + 1.2V.

Best Regards,
Rohit
• In reply to Rohit Bhat:

Hi Rohit-san,

I will explain more about the #2 question.

I  am concerned about the maximum rating of the input terminal of the ADC behind the LM6551Q-Q1.

The input absolute maximum rating voltage of ADC is 2.1V.

Since the gain of the amplifier is set according to a small input signal, it is assumed to clip with the output full swing voltage of the amplifier when the input signal becomes large.

I will adjust Vs so that the amplifier output does not exceed 2.1 V and as possible as wide.

I want to know the Vs voltage where the single ended output of the amplifier never exceeds 2.1V.

Then, ambient temperature is between -40 and 105deg.

Also, I would like to know the minimum and maximum output full swing voltage at that time.

# 3

Waveform is intended to be measured using the sample and the evaluation board, but it takes time to get it.

Could you take the waveform under the above conditions?

Behavior at the time of return becomes a very important factor.

Best regards,

Tomoaki Yoshida

• In reply to Tomoaki Yoshida:

Hi Tomoaki-san,

2. Do you know the ADC they are driving with the LMH6551Q-Q1? The output voltage swing interpretation of the LMH6551 can be found in the below diagram. On a single +3.3V supply, each of the single-ended output can swing maximum of 0.9Vpp or 1.8Vpp differential centered on a common mode voltage of 1.65V. If you look in the below diagram, the single-ended outputs swing 1.2V from either of the supply rails. So, +2.1V maximum Vout from the positive supply of +3.3V and +1.2V from the negative supply of GND (or 0V). You should be able to interpret the maximum output voltage swing in a similar fashion for a different power supply.

Is the input absolute maximum rating of 1.2V at the ADC single-ended or differential?

3. I will try to get the overdrive recovery data under Vs+ = 3.3V.

Best Regards,

Rohit

• In reply to Rohit Bhat:

Hi Rohit-san,

I could understan the output voltage swing interpretation of the LMH6551.

I want to tell the details directly by e-mail, so please email.

tomoaki_yoshida@nexty-ele.com

Best regards,

Tomoaki Yoshida