MC33078DT and input common mode voltage

[WuZunMing]

Edit by ST Moderator: This post has been translated from Chinese to comply with the ST Community Terms and Conditions (§10.2). The original can be found below.

Hello!

Material Model: MC33078DT, used in in-phase proportional amplification circuit, when the inverted end is grounded and the in-phase end is connected to 0.7V, abnormal: the output is close to the power supply voltage, when the inverted end is grounded and the in-phase end is connected to 0.8V, or greater than 0.8V, normal: the output is normal according to the amplification multiplier, please confirm whether it is related to the common mode voltage at the input?

If it is related to the input common mode voltage, since the specification does not specify the input common mode voltage range, please help me to confirm what is the input common mode voltage of this model? What is the output level below the input common mode voltage?

1. Single power supply VCC=5V, VCC-=0V:
   • When the in-phase V+ is connected to 0.7V or below, and the inverted V- is grounded, the output voltage VO is close to the power supply voltage of 5 V. At this time, the common mode voltage at the input is 0.35V and below.
   • When the same-phase terminal V+ is connected to 0.8V, or below 0.8V, the inverting terminal V- is grounded, and the output voltage VO is normally amplified by 4 times V+ (4V+). At this time the common mode voltage at the input is 0.4V and 0.4V or more.
2. Single-supply power supply VCC=15V, VCC-=0V:
   • When the in-phase V+ is connected to 0.7V or below, and the inverted V- is grounded, the output voltage VO is close to the power supply voltage of 15 V. At this time, the common-mode voltage at the input is 0.35V or below.
   • When V+ of the same phase terminal is connected to 0.8V, or below 0.8V, and V- of the inverting terminal is grounded, the output voltage VO is normally enlarged by 4 times V+ (4V+). At this time, the common mode voltage at the input is 0.4V and 0.4V or more.

您好！

物料型号：MC33078DT，用成同相比例放大电路，当反相端接地，同相端接0.7V时，异常：输出为接近电源电压，当反相端接地，同相端接0.8V时或大于0.8V，正常：输出按放大倍数正常输出，请确认是否与输入端共模电压有关？

如果与输入共模电压有关，因规格书未规定输入端共模电压范围，请帮忙确认此型号输入端共模电压是多少？低于输入端共模电压输出是什么电平？

1： 单电源供电VCC=5V，VCC-=0V：

1.1当同相端V+接入0.7V，或0.7V以下，反相端V-接地，输出电压VO接近电源电压5V。此时输入端的共模电压为0.35V及0.35V以下。

1.2当同相端V+接入0.8V，或0.8V以下，反相端V-接地，输出电压VO正常放大4倍V+（4V+）。此时输入端的共模电压为0.4V及0.4V以上。

2： 单电源供电VCC=15V，VCC-=0V：

1.1当同相端V+接入0.7V，或0.7V以下，反相端V-接地，输出电压VO接近电源电压15V。此时输入端的共模电压为0.35V及0.35V以下。

1.2当同相端V+接入0.8V，或0.8V以下，反相端V-接地，输出电压VO正常放大4倍V+（4V+）。此时输入端的共模电压为0.4V及0.4V以上。

[Peter BENSCH]

A picture is worth a thousand words: just add a schematic so that people can see what you are doing. It might then be possible to give you an answer, which is not possible now due to a lack of information on components in the vicinity of the MC33078.

Regards
/Peter

[WuZunMing]

Hello!

The schematic diagram is as below, on my side, I would like to know most of all, when the power supply of this model op-amp is a single power supply, what is the minimum input voltage of inverted end grounded and same phase end to work properly, thanks!

*This post has been translated from Chinese to comply with the ST Community guidelines.

[Peter BENSCH]

First of all, a note: the MC33078 came onto the market around 2000, when dual supply opamps were still predominantly used. Of course you can use any dual supply opamp with a single supply, but you have to pay attention to a few parameters.

One such parameter is the common mode voltage range, which is specified for a supply voltage of ±15V on the MC33078 (even if it is not explicitly stated in the data sheet) and has a range of (min) ±13V … (typ) ±14V. This results in a typical distance of 1V, maximum 2V to the rails, i.e. V+ and V-. You can therefore only feed the MC33078 with this input voltage of typically +1V, maximum +2V.

Does it answer your questions?

Regards
/Peter

[WuZunMing]

Greetings! May I ask, (you replied that one of the parameters is the common mode voltage range, which can only be used with an input voltage of usually +1V and max +2V for the MC33078). The input is normally +1V, does it mean (sum of in-phase input V+ and inverted input V-)/2>=1V, or does it mean that the in-phase input voltage >=1V or the inverted input voltage >=1V can be used, and if when the input voltage is not within the VCM range, then the in-phase op-amp amplifier function will be used as a comparator at this time?

*This post has been translated from Chinese to comply with the ST Community guidelines.

[Peter BENSCH]

According to my last statement, you can apply an input voltage of +2V in the schematics shown in your picture, which would be amplified to +8V with a factor of 4, which requires a VCC of at least 9.5V (RL=10kohms) or 10.8V (RL=600ohms). It may also work with an input voltage of +1V, but this is only a typical value and cannot be taken as a guaranteed value.

The function of such simple opamps is not guaranteed outside the specification, so that no statement can be made for an input voltage below 1V.

Regards
/Peter